diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 9bff305de..e80838a5e 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -12,7 +12,7 @@ jobs:
     strategy:
       matrix:
         os: [Ubuntu, MacOS, Windows]
-        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9.0-rc.1, pypy2, pypy3]
+        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9, pypy2, pypy3]
         exclude:
           - os: Ubuntu
             python-version: pypy2
@@ -48,6 +48,11 @@ jobs:
         shell: bash
         run: poetry config virtualenvs.in-project true
 
+      - name: Configure poetry installer
+        if: matrix.python-version == '2.7'
+        shell: bash
+        run: poetry config experimental.new-installer false
+
       - name: Set up cache
         uses: actions/cache@v1
         id: cache
diff --git a/poetry.lock b/poetry.lock
index 099974e5d..682e6ab72 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -27,17 +27,17 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
 [[package]]
 name = "attrs"
-version = "19.3.0"
+version = "20.3.0"
 description = "Classes Without Boilerplate"
 category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
 [package.extras]
-azure-pipelines = ["coverage", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "zope.interface", "pytest-azurepipelines"]
-dev = ["coverage", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "zope.interface", "sphinx", "pre-commit"]
-docs = ["sphinx", "zope.interface"]
-tests = ["coverage", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "zope.interface"]
+dev = ["coverage[toml] (>=5.0.2)", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "zope.interface", "furo", "sphinx", "pre-commit"]
+docs = ["furo", "sphinx", "zope.interface"]
+tests = ["coverage[toml] (>=5.0.2)", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "zope.interface"]
+tests_no_zope = ["coverage[toml] (>=5.0.2)", "hypothesis", "pympler", "pytest (>=4.3.0)", "six"]
 
 [[package]]
 name = "backports.functools-lru-cache"
@@ -49,7 +49,7 @@ python-versions = ">=2.6"
 
 [package.extras]
 docs = ["sphinx", "jaraco.packaging (>=3.2)", "rst.linker (>=1.9)"]
-testing = ["pytest (>=3.5,<3.7.3 || >3.7.3)", "pytest-checkdocs (>=1.2.3)", "pytest-flake8", "pytest-black-multipy", "pytest-cov"]
+testing = ["pytest (>=3.5,!=3.7.3)", "pytest-checkdocs (>=1.2.3)", "pytest-flake8", "pytest-black-multipy", "pytest-cov"]
 
 [[package]]
 name = "backports.tempfile"
@@ -72,7 +72,7 @@ python-versions = "*"
 
 [[package]]
 name = "certifi"
-version = "2020.6.20"
+version = "2020.12.5"
 description = "Python package for providing Mozilla's CA Bundle."
 category = "dev"
 optional = false
@@ -91,11 +91,11 @@ six = "*"
 
 [[package]]
 name = "chardet"
-version = "3.0.4"
+version = "4.0.0"
 description = "Universal encoding detector for Python 2 and 3"
 category = "dev"
 optional = false
-python-versions = "*"
+python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
 
 [[package]]
 name = "click"
@@ -107,7 +107,7 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
 
 [[package]]
 name = "colorama"
-version = "0.4.3"
+version = "0.4.4"
 description = "Cross-platform colored terminal text."
 category = "dev"
 optional = false
@@ -123,7 +123,7 @@ python-versions = ">=2.6"
 
 [package.extras]
 docs = ["sphinx", "jaraco.packaging (>=3.2)", "rst.linker (>=1.9)"]
-testing = ["pytest (>=3.5,<3.7.3 || >3.7.3)", "pytest-checkdocs (>=1.2)", "pytest-flake8", "pytest-black-multipy"]
+testing = ["pytest (>=3.5,!=3.7.3)", "pytest-checkdocs (>=1.2)", "pytest-flake8", "pytest-black-multipy"]
 
 [[package]]
 name = "contextlib2"
@@ -135,7 +135,7 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
 [[package]]
 name = "coverage"
-version = "5.2.1"
+version = "5.3.1"
 description = "Code coverage measurement for Python"
 category = "dev"
 optional = false
@@ -194,7 +194,7 @@ python-versions = ">=2.6, <3"
 
 [[package]]
 name = "identify"
-version = "1.4.26"
+version = "1.5.13"
 description = "File identification library for Python"
 category = "dev"
 optional = false
@@ -219,27 +219,24 @@ category = "main"
 optional = false
 python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,>=2.7"
 
-[package.extras]
-docs = ["sphinx", "rst.linker"]
-testing = ["packaging", "pep517", "importlib-resources (>=1.3)"]
-
 [package.dependencies]
-zipp = ">=0.5"
 configparser = {version = ">=3.5", markers = "python_version < \"3\""}
 contextlib2 = {version = "*", markers = "python_version < \"3\""}
 pathlib2 = {version = "*", markers = "python_version < \"3\""}
+zipp = ">=0.5"
+
+[package.extras]
+docs = ["sphinx", "rst.linker"]
+testing = ["packaging", "pep517", "importlib-resources (>=1.3)"]
 
 [[package]]
 name = "importlib-resources"
-version = "3.0.0"
+version = "3.2.1"
 description = "Read resources from Python packages"
 category = "dev"
 optional = false
 python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,>=2.7"
 
-[package.extras]
-docs = ["sphinx", "rst.linker", "jaraco.packaging"]
-
 [package.dependencies]
 contextlib2 = {version = "*", markers = "python_version < \"3\""}
 pathlib2 = {version = "*", markers = "python_version < \"3\""}
@@ -247,6 +244,9 @@ singledispatch = {version = "*", markers = "python_version < \"3.4\""}
 typing = {version = "*", markers = "python_version < \"3.5\""}
 zipp = {version = ">=0.4", markers = "python_version < \"3.8\""}
 
+[package.extras]
+docs = ["sphinx", "rst.linker", "jaraco.packaging"]
+
 [[package]]
 name = "jsonschema"
 version = "3.2.0"
@@ -255,15 +255,15 @@ category = "dev"
 optional = false
 python-versions = "*"
 
-[package.extras]
-format = ["idna", "jsonpointer (>1.13)", "rfc3987", "strict-rfc3339", "webcolors"]
-format_nongpl = ["idna", "jsonpointer (>1.13)", "webcolors", "rfc3986-validator (>0.1.0)", "rfc3339-validator"]
-
 [package.dependencies]
 attrs = ">=17.4.0"
 pyrsistent = ">=0.14.0"
 six = ">=1.11.0"
 
+[package.extras]
+format = ["idna", "jsonpointer (>1.13)", "rfc3987", "strict-rfc3339", "webcolors"]
+format_nongpl = ["idna", "jsonpointer (>1.13)", "webcolors", "rfc3986-validator (>0.1.0)", "rfc3339-validator"]
+
 [[package]]
 name = "mock"
 version = "3.0.5"
@@ -272,15 +272,15 @@ category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
+[package.dependencies]
+funcsigs = {version = ">=1", markers = "python_version < \"3.3\""}
+six = "*"
+
 [package.extras]
 build = ["twine", "wheel", "blurb"]
 docs = ["sphinx"]
 test = ["pytest", "pytest-cov"]
 
-[package.dependencies]
-six = "*"
-funcsigs = {version = ">=1", markers = "python_version < \"3.3\""}
-
 [[package]]
 name = "more-itertools"
 version = "5.0.0"
@@ -294,7 +294,7 @@ six = ">=1.0.0,<2.0.0"
 
 [[package]]
 name = "more-itertools"
-version = "8.4.0"
+version = "8.6.0"
 description = "More routines for operating on iterables, beyond itertools"
 category = "dev"
 optional = false
@@ -302,7 +302,7 @@ python-versions = ">=3.5"
 
 [[package]]
 name = "nodeenv"
-version = "1.4.0"
+version = "1.5.0"
 description = "Node.js virtual environment builder"
 category = "dev"
 optional = false
@@ -310,7 +310,7 @@ python-versions = "*"
 
 [[package]]
 name = "packaging"
-version = "20.4"
+version = "20.8"
 description = "Core utilities for Python packages"
 category = "dev"
 optional = false
@@ -318,7 +318,6 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
 [package.dependencies]
 pyparsing = ">=2.0.2"
-six = "*"
 
 [[package]]
 name = "pathlib2"
@@ -329,8 +328,8 @@ optional = false
 python-versions = "*"
 
 [package.dependencies]
-six = "*"
 scandir = {version = "*", markers = "python_version < \"3.5\""}
+six = "*"
 
 [[package]]
 name = "pep517"
@@ -341,8 +340,8 @@ optional = false
 python-versions = "*"
 
 [package.dependencies]
-toml = "*"
 importlib_metadata = {version = "*", markers = "python_version < \"3.8\""}
+toml = "*"
 zipp = {version = "*", markers = "python_version < \"3.8\""}
 
 [[package]]
@@ -353,12 +352,12 @@ category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
-[package.extras]
-dev = ["pre-commit", "tox"]
-
 [package.dependencies]
 importlib-metadata = {version = ">=0.12", markers = "python_version < \"3.8\""}
 
+[package.extras]
+dev = ["pre-commit", "tox"]
+
 [[package]]
 name = "pre-commit"
 version = "1.21.0"
@@ -370,19 +369,19 @@ python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,>=2.7"
 [package.dependencies]
 "aspy.yaml" = "*"
 cfgv = ">=2.0.0"
+futures = {version = "*", markers = "python_version < \"3.2\""}
 identify = ">=1.0.0"
+importlib-metadata = {version = "*", markers = "python_version < \"3.8\""}
+importlib-resources = {version = "*", markers = "python_version < \"3.7\""}
 nodeenv = ">=0.11.1"
 pyyaml = "*"
 six = "*"
 toml = "*"
 virtualenv = ">=15.2"
-futures = {version = "*", markers = "python_version < \"3.2\""}
-importlib-metadata = {version = "*", markers = "python_version < \"3.8\""}
-importlib-resources = {version = "*", markers = "python_version < \"3.7\""}
 
 [[package]]
 name = "py"
-version = "1.9.0"
+version = "1.10.0"
 description = "library with cross-python path, ini-parsing, io, code, log facilities"
 category = "dev"
 optional = false
@@ -398,11 +397,11 @@ python-versions = ">=2.6, !=3.0.*, !=3.1.*, !=3.2.*"
 
 [[package]]
 name = "pyrsistent"
-version = "0.16.0"
+version = "0.16.1"
 description = "Persistent/Functional/Immutable data structures"
 category = "dev"
 optional = false
-python-versions = "*"
+python-versions = ">=2.7"
 
 [package.dependencies]
 six = "*"
@@ -415,45 +414,41 @@ category = "dev"
 optional = false
 python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,>=2.7"
 
-[package.extras]
-testing = ["argcomplete", "hypothesis (>=3.56)", "nose", "requests", "mock"]
-
 [package.dependencies]
 atomicwrites = ">=1.0"
 attrs = ">=17.4.0"
+colorama = {version = "*", markers = "sys_platform == \"win32\" and python_version != \"3.4\""}
+funcsigs = {version = ">=1.0", markers = "python_version < \"3.0\""}
+importlib-metadata = {version = ">=0.12", markers = "python_version < \"3.8\""}
+more-itertools = [
+    {version = ">=4.0.0,<6.0.0", markers = "python_version <= \"2.7\""},
+    {version = ">=4.0.0", markers = "python_version > \"2.7\""},
+]
 packaging = "*"
+pathlib2 = {version = ">=2.2.0", markers = "python_version < \"3.6\""}
 pluggy = ">=0.12,<1.0"
 py = ">=1.5.0"
 six = ">=1.10.0"
 wcwidth = "*"
-colorama = {version = "*", markers = "sys_platform == \"win32\" and python_version != \"3.4\""}
-funcsigs = {version = ">=1.0", markers = "python_version < \"3.0\""}
-importlib-metadata = {version = ">=0.12", markers = "python_version < \"3.8\""}
-pathlib2 = {version = ">=2.2.0", markers = "python_version < \"3.6\""}
-
-[[package.dependencies.more-itertools]]
-version = ">=4.0.0,<6.0.0"
-markers = "python_version <= \"2.7\""
 
-[[package.dependencies.more-itertools]]
-version = ">=4.0.0"
-markers = "python_version > \"2.7\""
+[package.extras]
+testing = ["argcomplete", "hypothesis (>=3.56)", "nose", "requests", "mock"]
 
 [[package]]
 name = "pytest-cov"
-version = "2.10.1"
+version = "2.11.0"
 description = "Pytest plugin for measuring coverage."
 category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
 
-[package.extras]
-testing = ["fields", "hunter", "process-tests (2.0.2)", "six", "pytest-xdist", "virtualenv"]
-
 [package.dependencies]
-coverage = ">=4.4"
+coverage = ">=5.2.1"
 pytest = ">=4.6"
 
+[package.extras]
+testing = ["fields", "hunter", "process-tests (==2.0.2)", "six", "pytest-xdist", "virtualenv"]
+
 [[package]]
 name = "pytest-mock"
 version = "2.0.0"
@@ -462,12 +457,12 @@ category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
-[package.extras]
-dev = ["pre-commit", "tox"]
-
 [package.dependencies]
-pytest = ">=2.7"
 mock = {version = "*", markers = "python_version < \"3.0\""}
+pytest = ">=2.7"
+
+[package.extras]
+dev = ["pre-commit", "tox"]
 
 [[package]]
 name = "pyyaml"
@@ -479,21 +474,21 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
 
 [[package]]
 name = "requests"
-version = "2.24.0"
+version = "2.25.1"
 description = "Python HTTP for Humans."
 category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
 
-[package.extras]
-security = ["pyOpenSSL (>=0.14)", "cryptography (>=1.3.4)"]
-socks = ["PySocks (>=1.5.6,<1.5.7 || >1.5.7)", "win-inet-pton"]
-
 [package.dependencies]
 certifi = ">=2017.4.17"
-chardet = ">=3.0.2,<4"
+chardet = ">=3.0.2,<5"
 idna = ">=2.5,<3"
-urllib3 = ">=1.21.1,<1.25.0 || >1.25.0,<1.25.1 || >1.25.1,<1.26"
+urllib3 = ">=1.21.1,<1.27"
+
+[package.extras]
+security = ["pyOpenSSL (>=0.14)", "cryptography (>=1.3.4)"]
+socks = ["PySocks (>=1.5.6,!=1.5.7)", "win-inet-pton"]
 
 [[package]]
 name = "scandir"
@@ -524,34 +519,34 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*"
 
 [[package]]
 name = "toml"
-version = "0.10.1"
+version = "0.10.2"
 description = "Python Library for Tom's Obvious, Minimal Language"
 category = "dev"
 optional = false
-python-versions = "*"
+python-versions = ">=2.6, !=3.0.*, !=3.1.*, !=3.2.*"
 
 [[package]]
 name = "tox"
-version = "3.19.0"
+version = "3.21.1"
 description = "tox is a generic virtualenv management and test command line tool"
 category = "dev"
 optional = false
 python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,>=2.7"
 
-[package.extras]
-docs = ["pygments-github-lexers (>=0.0.5)", "sphinx (>=2.0.0)", "sphinxcontrib-autoprogram (>=0.1.5)", "towncrier (>=18.5.0)"]
-testing = ["flaky (>=3.4.0)", "freezegun (>=0.3.11)", "pathlib2 (>=2.3.3)", "psutil (>=5.6.1)", "pytest (>=4.0.0)", "pytest-cov (>=2.5.1)", "pytest-mock (>=1.10.0)", "pytest-randomly (>=1.0.0)", "pytest-xdist (>=1.22.2)"]
-
 [package.dependencies]
+colorama = {version = ">=0.4.1", markers = "platform_system == \"Windows\""}
 filelock = ">=3.0.0"
+importlib-metadata = {version = ">=0.12", markers = "python_version < \"3.8\""}
 packaging = ">=14"
 pluggy = ">=0.12.0"
 py = ">=1.4.17"
 six = ">=1.14.0"
 toml = ">=0.9.4"
 virtualenv = ">=16.0.0,<20.0.0 || >20.0.0,<20.0.1 || >20.0.1,<20.0.2 || >20.0.2,<20.0.3 || >20.0.3,<20.0.4 || >20.0.4,<20.0.5 || >20.0.5,<20.0.6 || >20.0.6,<20.0.7 || >20.0.7"
-colorama = {version = ">=0.4.1", markers = "platform_system == \"Windows\""}
-importlib-metadata = {version = ">=0.12,<2", markers = "python_version < \"3.8\""}
+
+[package.extras]
+docs = ["pygments-github-lexers (>=0.0.5)", "sphinx (>=2.0.0)", "sphinxcontrib-autoprogram (>=0.1.5)", "towncrier (>=18.5.0)"]
+testing = ["flaky (>=3.4.0)", "freezegun (>=0.3.11)", "psutil (>=5.6.1)", "pytest (>=4.0.0)", "pytest-cov (>=2.5.1)", "pytest-mock (>=1.10.0)", "pytest-randomly (>=1.0.0)", "pytest-xdist (>=1.22.2)", "pathlib2 (>=2.3.3)"]
 
 [[package]]
 name = "typing"
@@ -563,7 +558,7 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
 [[package]]
 name = "urllib3"
-version = "1.25.10"
+version = "1.26.2"
 description = "HTTP library with thread-safe connection pooling, file post, and more."
 category = "dev"
 optional = false
@@ -571,20 +566,16 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, <4"
 
 [package.extras]
 brotli = ["brotlipy (>=0.6.0)"]
-secure = ["certifi", "cryptography (>=1.3.4)", "idna (>=2.0.0)", "pyOpenSSL (>=0.14)", "ipaddress"]
-socks = ["PySocks (>=1.5.6,<1.5.7 || >1.5.7,<2.0)"]
+secure = ["pyOpenSSL (>=0.14)", "cryptography (>=1.3.4)", "idna (>=2.0.0)", "certifi", "ipaddress"]
+socks = ["PySocks (>=1.5.6,!=1.5.7,<2.0)"]
 
 [[package]]
 name = "vendoring"
-version = "0.3.2"
+version = "0.3.3"
 description = "A command line tool, to simplify vendoring pure Python dependencies."
 category = "dev"
 optional = false
-python-versions = "~= 3.8.0"
-
-[package.extras]
-doc = ["sphinx"]
-test = ["pytest", "pytest-xdist", "pytest-cov", "pytest-mock"]
+python-versions = "~= 3.8"
 
 [package.dependencies]
 click = "*"
@@ -593,26 +584,30 @@ packaging = "*"
 requests = "*"
 toml = "*"
 
+[package.extras]
+doc = ["sphinx"]
+test = ["pytest", "pytest-xdist", "pytest-cov", "pytest-mock"]
+
 [[package]]
 name = "virtualenv"
-version = "20.0.30"
+version = "20.3.1"
 description = "Virtual Python Environment builder"
 category = "dev"
 optional = false
 python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,>=2.7"
 
-[package.extras]
-docs = ["proselint (>=0.10.2)", "sphinx (>=3)", "sphinx-argparse (>=0.2.5)", "sphinx-rtd-theme (>=0.4.3)", "towncrier (>=19.9.0rc1)"]
-testing = ["coverage (>=5)", "coverage-enable-subprocess (>=1)", "flaky (>=3)", "pytest (>=4)", "pytest-env (>=0.6.2)", "pytest-freezegun (>=0.4.1)", "pytest-mock (>=2)", "pytest-randomly (>=1)", "pytest-timeout (>=1)", "pytest-xdist (>=1.31.0)", "packaging (>=20.0)", "xonsh (>=0.9.16)"]
-
 [package.dependencies]
 appdirs = ">=1.4.3,<2"
 distlib = ">=0.3.1,<1"
 filelock = ">=3.0.0,<4"
-six = ">=1.9.0,<2"
-importlib-metadata = {version = ">=0.12,<2", markers = "python_version < \"3.8\""}
+importlib-metadata = {version = ">=0.12", markers = "python_version < \"3.8\""}
 importlib-resources = {version = ">=1.0", markers = "python_version < \"3.7\""}
 pathlib2 = {version = ">=2.3.3,<3", markers = "python_version < \"3.4\" and sys_platform != \"win32\""}
+six = ">=1.9.0,<2"
+
+[package.extras]
+docs = ["proselint (>=0.10.2)", "sphinx (>=3)", "sphinx-argparse (>=0.2.5)", "sphinx-rtd-theme (>=0.4.3)", "towncrier (>=19.9.0rc1)"]
+testing = ["coverage (>=4)", "coverage-enable-subprocess (>=1)", "flaky (>=3)", "pytest (>=4)", "pytest-env (>=0.6.2)", "pytest-freezegun (>=0.4.1)", "pytest-mock (>=2)", "pytest-randomly (>=1)", "pytest-timeout (>=1)", "packaging (>=20.0)", "xonsh (>=0.9.16)"]
 
 [[package]]
 name = "wcwidth"
@@ -633,17 +628,17 @@ category = "main"
 optional = false
 python-versions = ">=2.7"
 
+[package.dependencies]
+contextlib2 = {version = "*", markers = "python_version < \"3.4\""}
+
 [package.extras]
 docs = ["sphinx", "jaraco.packaging (>=3.2)", "rst.linker (>=1.9)"]
 testing = ["pathlib2", "unittest2", "jaraco.itertools", "func-timeout"]
 
-[package.dependencies]
-contextlib2 = {version = "*", markers = "python_version < \"3.4\""}
-
 [metadata]
 lock-version = "1.1"
 python-versions = "~2.7 || ^3.5"
-content-hash = "6e56f24c806c05b85548fd96b962ed2b0d2b53e8723d382de9118e788b63323d"
+content-hash = "73b0c1d12930e6381fb1a47e8c903603ac18a2981ee899f9f675c4ebcbbbe3ff"
 
 [metadata.files]
 appdirs = [
@@ -659,8 +654,8 @@ atomicwrites = [
     {file = "atomicwrites-1.4.0.tar.gz", hash = "sha256:ae70396ad1a434f9c7046fd2dd196fc04b12f9e91ffb859164193be8b6168a7a"},
 ]
 attrs = [
-    {file = "attrs-19.3.0-py2.py3-none-any.whl", hash = "sha256:08a96c641c3a74e44eb59afb61a24f2cb9f4d7188748e76ba4bb5edfa3cb7d1c"},
-    {file = "attrs-19.3.0.tar.gz", hash = "sha256:f7b7ce16570fe9965acd6d30101a28f62fb4a7f9e926b3bbc9b61f8b04247e72"},
+    {file = "attrs-20.3.0-py2.py3-none-any.whl", hash = "sha256:31b2eced602aa8423c2aea9c76a724617ed67cf9513173fd3a4f03e3a929c7e6"},
+    {file = "attrs-20.3.0.tar.gz", hash = "sha256:832aa3cde19744e49938b91fea06d69ecb9e649c93ba974535d08ad92164f700"},
 ]
 "backports.functools-lru-cache" = [
     {file = "backports.functools_lru_cache-1.6.1-py2.py3-none-any.whl", hash = "sha256:0bada4c2f8a43d533e4ecb7a12214d9420e66eb206d54bf2d682581ca4b80848"},
@@ -675,24 +670,24 @@ attrs = [
     {file = "backports.weakref-1.0.post1.tar.gz", hash = "sha256:bc4170a29915f8b22c9e7c4939701859650f2eb84184aee80da329ac0b9825c2"},
 ]
 certifi = [
-    {file = "certifi-2020.6.20-py2.py3-none-any.whl", hash = "sha256:8fc0819f1f30ba15bdb34cceffb9ef04d99f420f68eb75d901e9560b8749fc41"},
-    {file = "certifi-2020.6.20.tar.gz", hash = "sha256:5930595817496dd21bb8dc35dad090f1c2cd0adfaf21204bf6732ca5d8ee34d3"},
+    {file = "certifi-2020.12.5-py2.py3-none-any.whl", hash = "sha256:719a74fb9e33b9bd44cc7f3a8d94bc35e4049deebe19ba7d8e108280cfd59830"},
+    {file = "certifi-2020.12.5.tar.gz", hash = "sha256:1a4995114262bffbc2413b159f2a1a480c969de6e6eb13ee966d470af86af59c"},
 ]
 cfgv = [
     {file = "cfgv-2.0.1-py2.py3-none-any.whl", hash = "sha256:fbd93c9ab0a523bf7daec408f3be2ed99a980e20b2d19b50fc184ca6b820d289"},
     {file = "cfgv-2.0.1.tar.gz", hash = "sha256:edb387943b665bf9c434f717bf630fa78aecd53d5900d2e05da6ad6048553144"},
 ]
 chardet = [
-    {file = "chardet-3.0.4-py2.py3-none-any.whl", hash = "sha256:fc323ffcaeaed0e0a02bf4d117757b98aed530d9ed4531e3e15460124c106691"},
-    {file = "chardet-3.0.4.tar.gz", hash = "sha256:84ab92ed1c4d4f16916e05906b6b75a6c0fb5db821cc65e70cbd64a3e2a5eaae"},
+    {file = "chardet-4.0.0-py2.py3-none-any.whl", hash = "sha256:f864054d66fd9118f2e67044ac8981a54775ec5b67aed0441892edb553d21da5"},
+    {file = "chardet-4.0.0.tar.gz", hash = "sha256:0d6f53a15db4120f2b08c94f11e7d93d2c911ee118b6b30a04ec3ee8310179fa"},
 ]
 click = [
     {file = "click-7.1.2-py2.py3-none-any.whl", hash = "sha256:dacca89f4bfadd5de3d7489b7c8a566eee0d3676333fbb50030263894c38c0dc"},
     {file = "click-7.1.2.tar.gz", hash = "sha256:d2b5255c7c6349bc1bd1e59e08cd12acbbd63ce649f2588755783aa94dfb6b1a"},
 ]
 colorama = [
-    {file = "colorama-0.4.3-py2.py3-none-any.whl", hash = "sha256:7d73d2a99753107a36ac6b455ee49046802e59d9d076ef8e47b61499fa29afff"},
-    {file = "colorama-0.4.3.tar.gz", hash = "sha256:e96da0d330793e2cb9485e9ddfd918d456036c7149416295932478192f4436a1"},
+    {file = "colorama-0.4.4-py2.py3-none-any.whl", hash = "sha256:9f47eda37229f68eee03b24b9748937c7dc3868f906e8ba69fbcbdd3bc5dc3e2"},
+    {file = "colorama-0.4.4.tar.gz", hash = "sha256:5941b2b48a20143d2267e95b1c2a7603ce057ee39fd88e7329b0c292aa16869b"},
 ]
 configparser = [
     {file = "configparser-4.0.2-py2.py3-none-any.whl", hash = "sha256:254c1d9c79f60c45dfde850850883d5aaa7f19a23f13561243a050d5a7c3fe4c"},
@@ -703,40 +698,55 @@ contextlib2 = [
     {file = "contextlib2-0.6.0.post1.tar.gz", hash = "sha256:01f490098c18b19d2bd5bb5dc445b2054d2fa97f09a4280ba2c5f3c394c8162e"},
 ]
 coverage = [
-    {file = "coverage-5.2.1-cp27-cp27m-macosx_10_13_intel.whl", hash = "sha256:40f70f81be4d34f8d491e55936904db5c527b0711b2a46513641a5729783c2e4"},
-    {file = "coverage-5.2.1-cp27-cp27m-macosx_10_9_x86_64.whl", hash = "sha256:675192fca634f0df69af3493a48224f211f8db4e84452b08d5fcebb9167adb01"},
-    {file = "coverage-5.2.1-cp27-cp27m-manylinux1_i686.whl", hash = "sha256:2fcc8b58953d74d199a1a4d633df8146f0ac36c4e720b4a1997e9b6327af43a8"},
-    {file = "coverage-5.2.1-cp27-cp27m-manylinux1_x86_64.whl", hash = "sha256:64c4f340338c68c463f1b56e3f2f0423f7b17ba6c3febae80b81f0e093077f59"},
-    {file = "coverage-5.2.1-cp27-cp27m-win32.whl", hash = "sha256:52f185ffd3291196dc1aae506b42e178a592b0b60a8610b108e6ad892cfc1bb3"},
-    {file = "coverage-5.2.1-cp27-cp27m-win_amd64.whl", hash = "sha256:30bc103587e0d3df9e52cd9da1dd915265a22fad0b72afe54daf840c984b564f"},
-    {file = "coverage-5.2.1-cp27-cp27mu-manylinux1_i686.whl", hash = "sha256:9ea749fd447ce7fb1ac71f7616371f04054d969d412d37611716721931e36efd"},
-    {file = "coverage-5.2.1-cp27-cp27mu-manylinux1_x86_64.whl", hash = "sha256:ce7866f29d3025b5b34c2e944e66ebef0d92e4a4f2463f7266daa03a1332a651"},
-    {file = "coverage-5.2.1-cp35-cp35m-macosx_10_13_x86_64.whl", hash = "sha256:4869ab1c1ed33953bb2433ce7b894a28d724b7aa76c19b11e2878034a4e4680b"},
-    {file = "coverage-5.2.1-cp35-cp35m-manylinux1_i686.whl", hash = "sha256:a3ee9c793ffefe2944d3a2bd928a0e436cd0ac2d9e3723152d6fd5398838ce7d"},
-    {file = "coverage-5.2.1-cp35-cp35m-manylinux1_x86_64.whl", hash = "sha256:28f42dc5172ebdc32622a2c3f7ead1b836cdbf253569ae5673f499e35db0bac3"},
-    {file = "coverage-5.2.1-cp35-cp35m-win32.whl", hash = "sha256:e26c993bd4b220429d4ec8c1468eca445a4064a61c74ca08da7429af9bc53bb0"},
-    {file = "coverage-5.2.1-cp35-cp35m-win_amd64.whl", hash = "sha256:4186fc95c9febeab5681bc3248553d5ec8c2999b8424d4fc3a39c9cba5796962"},
-    {file = "coverage-5.2.1-cp36-cp36m-macosx_10_13_x86_64.whl", hash = "sha256:b360d8fd88d2bad01cb953d81fd2edd4be539df7bfec41e8753fe9f4456a5082"},
-    {file = "coverage-5.2.1-cp36-cp36m-manylinux1_i686.whl", hash = "sha256:1adb6be0dcef0cf9434619d3b892772fdb48e793300f9d762e480e043bd8e716"},
-    {file = "coverage-5.2.1-cp36-cp36m-manylinux1_x86_64.whl", hash = "sha256:098a703d913be6fbd146a8c50cc76513d726b022d170e5e98dc56d958fd592fb"},
-    {file = "coverage-5.2.1-cp36-cp36m-win32.whl", hash = "sha256:962c44070c281d86398aeb8f64e1bf37816a4dfc6f4c0f114756b14fc575621d"},
-    {file = "coverage-5.2.1-cp36-cp36m-win_amd64.whl", hash = "sha256:b1ed2bdb27b4c9fc87058a1cb751c4df8752002143ed393899edb82b131e0546"},
-    {file = "coverage-5.2.1-cp37-cp37m-macosx_10_13_x86_64.whl", hash = "sha256:c890728a93fffd0407d7d37c1e6083ff3f9f211c83b4316fae3778417eab9811"},
-    {file = "coverage-5.2.1-cp37-cp37m-manylinux1_i686.whl", hash = "sha256:538f2fd5eb64366f37c97fdb3077d665fa946d2b6d95447622292f38407f9258"},
-    {file = "coverage-5.2.1-cp37-cp37m-manylinux1_x86_64.whl", hash = "sha256:27ca5a2bc04d68f0776f2cdcb8bbd508bbe430a7bf9c02315cd05fb1d86d0034"},
-    {file = "coverage-5.2.1-cp37-cp37m-win32.whl", hash = "sha256:aab75d99f3f2874733946a7648ce87a50019eb90baef931698f96b76b6769a46"},
-    {file = "coverage-5.2.1-cp37-cp37m-win_amd64.whl", hash = "sha256:c2ff24df02a125b7b346c4c9078c8936da06964cc2d276292c357d64378158f8"},
-    {file = "coverage-5.2.1-cp38-cp38-macosx_10_9_x86_64.whl", hash = "sha256:304fbe451698373dc6653772c72c5d5e883a4aadaf20343592a7abb2e643dae0"},
-    {file = "coverage-5.2.1-cp38-cp38-manylinux1_i686.whl", hash = "sha256:c96472b8ca5dc135fb0aa62f79b033f02aa434fb03a8b190600a5ae4102df1fd"},
-    {file = "coverage-5.2.1-cp38-cp38-manylinux1_x86_64.whl", hash = "sha256:8505e614c983834239f865da2dd336dcf9d72776b951d5dfa5ac36b987726e1b"},
-    {file = "coverage-5.2.1-cp38-cp38-win32.whl", hash = "sha256:700997b77cfab016533b3e7dbc03b71d33ee4df1d79f2463a318ca0263fc29dd"},
-    {file = "coverage-5.2.1-cp38-cp38-win_amd64.whl", hash = "sha256:46794c815e56f1431c66d81943fa90721bb858375fb36e5903697d5eef88627d"},
-    {file = "coverage-5.2.1-cp39-cp39-macosx_10_13_x86_64.whl", hash = "sha256:16042dc7f8e632e0dcd5206a5095ebd18cb1d005f4c89694f7f8aafd96dd43a3"},
-    {file = "coverage-5.2.1-cp39-cp39-manylinux1_i686.whl", hash = "sha256:c1bbb628ed5192124889b51204de27c575b3ffc05a5a91307e7640eff1d48da4"},
-    {file = "coverage-5.2.1-cp39-cp39-manylinux1_x86_64.whl", hash = "sha256:4f6428b55d2916a69f8d6453e48a505c07b2245653b0aa9f0dee38785939f5e4"},
-    {file = "coverage-5.2.1-cp39-cp39-win32.whl", hash = "sha256:9e536783a5acee79a9b308be97d3952b662748c4037b6a24cbb339dc7ed8eb89"},
-    {file = "coverage-5.2.1-cp39-cp39-win_amd64.whl", hash = "sha256:b8f58c7db64d8f27078cbf2a4391af6aa4e4767cc08b37555c4ae064b8558d9b"},
-    {file = "coverage-5.2.1.tar.gz", hash = "sha256:a34cb28e0747ea15e82d13e14de606747e9e484fb28d63c999483f5d5188e89b"},
+    {file = "coverage-5.3.1-cp27-cp27m-macosx_10_9_x86_64.whl", hash = "sha256:fabeeb121735d47d8eab8671b6b031ce08514c86b7ad8f7d5490a7b6dcd6267d"},
+    {file = "coverage-5.3.1-cp27-cp27m-manylinux1_i686.whl", hash = "sha256:7e4d159021c2029b958b2363abec4a11db0ce8cd43abb0d9ce44284cb97217e7"},
+    {file = "coverage-5.3.1-cp27-cp27m-manylinux1_x86_64.whl", hash = "sha256:378ac77af41350a8c6b8801a66021b52da8a05fd77e578b7380e876c0ce4f528"},
+    {file = "coverage-5.3.1-cp27-cp27m-manylinux2010_i686.whl", hash = "sha256:e448f56cfeae7b1b3b5bcd99bb377cde7c4eb1970a525c770720a352bc4c8044"},
+    {file = "coverage-5.3.1-cp27-cp27m-manylinux2010_x86_64.whl", hash = "sha256:cc44e3545d908ecf3e5773266c487ad1877be718d9dc65fc7eb6e7d14960985b"},
+    {file = "coverage-5.3.1-cp27-cp27m-win32.whl", hash = "sha256:08b3ba72bd981531fd557f67beee376d6700fba183b167857038997ba30dd297"},
+    {file = "coverage-5.3.1-cp27-cp27m-win_amd64.whl", hash = "sha256:8dacc4073c359f40fcf73aede8428c35f84639baad7e1b46fce5ab7a8a7be4bb"},
+    {file = "coverage-5.3.1-cp27-cp27mu-manylinux1_i686.whl", hash = "sha256:ee2f1d1c223c3d2c24e3afbb2dd38be3f03b1a8d6a83ee3d9eb8c36a52bee899"},
+    {file = "coverage-5.3.1-cp27-cp27mu-manylinux1_x86_64.whl", hash = "sha256:9a9d4ff06804920388aab69c5ea8a77525cf165356db70131616acd269e19b36"},
+    {file = "coverage-5.3.1-cp27-cp27mu-manylinux2010_i686.whl", hash = "sha256:782a5c7df9f91979a7a21792e09b34a658058896628217ae6362088b123c8500"},
+    {file = "coverage-5.3.1-cp27-cp27mu-manylinux2010_x86_64.whl", hash = "sha256:fda29412a66099af6d6de0baa6bd7c52674de177ec2ad2630ca264142d69c6c7"},
+    {file = "coverage-5.3.1-cp35-cp35m-macosx_10_9_x86_64.whl", hash = "sha256:f2c6888eada180814b8583c3e793f3f343a692fc802546eed45f40a001b1169f"},
+    {file = "coverage-5.3.1-cp35-cp35m-manylinux1_i686.whl", hash = "sha256:8f33d1156241c43755137288dea619105477961cfa7e47f48dbf96bc2c30720b"},
+    {file = "coverage-5.3.1-cp35-cp35m-manylinux1_x86_64.whl", hash = "sha256:b239711e774c8eb910e9b1ac719f02f5ae4bf35fa0420f438cdc3a7e4e7dd6ec"},
+    {file = "coverage-5.3.1-cp35-cp35m-manylinux2010_i686.whl", hash = "sha256:f54de00baf200b4539a5a092a759f000b5f45fd226d6d25a76b0dff71177a714"},
+    {file = "coverage-5.3.1-cp35-cp35m-manylinux2010_x86_64.whl", hash = "sha256:be0416074d7f253865bb67630cf7210cbc14eb05f4099cc0f82430135aaa7a3b"},
+    {file = "coverage-5.3.1-cp35-cp35m-win32.whl", hash = "sha256:c46643970dff9f5c976c6512fd35768c4a3819f01f61169d8cdac3f9290903b7"},
+    {file = "coverage-5.3.1-cp35-cp35m-win_amd64.whl", hash = "sha256:9a4f66259bdd6964d8cf26142733c81fb562252db74ea367d9beb4f815478e72"},
+    {file = "coverage-5.3.1-cp36-cp36m-macosx_10_9_x86_64.whl", hash = "sha256:c6e5174f8ca585755988bc278c8bb5d02d9dc2e971591ef4a1baabdf2d99589b"},
+    {file = "coverage-5.3.1-cp36-cp36m-manylinux1_i686.whl", hash = "sha256:3911c2ef96e5ddc748a3c8b4702c61986628bb719b8378bf1e4a6184bbd48fe4"},
+    {file = "coverage-5.3.1-cp36-cp36m-manylinux1_x86_64.whl", hash = "sha256:c5ec71fd4a43b6d84ddb88c1df94572479d9a26ef3f150cef3dacefecf888105"},
+    {file = "coverage-5.3.1-cp36-cp36m-manylinux2010_i686.whl", hash = "sha256:f51dbba78d68a44e99d484ca8c8f604f17e957c1ca09c3ebc2c7e3bbd9ba0448"},
+    {file = "coverage-5.3.1-cp36-cp36m-manylinux2010_x86_64.whl", hash = "sha256:a2070c5affdb3a5e751f24208c5c4f3d5f008fa04d28731416e023c93b275277"},
+    {file = "coverage-5.3.1-cp36-cp36m-win32.whl", hash = "sha256:535dc1e6e68fad5355f9984d5637c33badbdc987b0c0d303ee95a6c979c9516f"},
+    {file = "coverage-5.3.1-cp36-cp36m-win_amd64.whl", hash = "sha256:a4857f7e2bc6921dbd487c5c88b84f5633de3e7d416c4dc0bb70256775551a6c"},
+    {file = "coverage-5.3.1-cp37-cp37m-macosx_10_9_x86_64.whl", hash = "sha256:fac3c432851038b3e6afe086f777732bcf7f6ebbfd90951fa04ee53db6d0bcdd"},
+    {file = "coverage-5.3.1-cp37-cp37m-manylinux1_i686.whl", hash = "sha256:cd556c79ad665faeae28020a0ab3bda6cd47d94bec48e36970719b0b86e4dcf4"},
+    {file = "coverage-5.3.1-cp37-cp37m-manylinux1_x86_64.whl", hash = "sha256:a66ca3bdf21c653e47f726ca57f46ba7fc1f260ad99ba783acc3e58e3ebdb9ff"},
+    {file = "coverage-5.3.1-cp37-cp37m-manylinux2010_i686.whl", hash = "sha256:ab110c48bc3d97b4d19af41865e14531f300b482da21783fdaacd159251890e8"},
+    {file = "coverage-5.3.1-cp37-cp37m-manylinux2010_x86_64.whl", hash = "sha256:e52d3d95df81c8f6b2a1685aabffadf2d2d9ad97203a40f8d61e51b70f191e4e"},
+    {file = "coverage-5.3.1-cp37-cp37m-win32.whl", hash = "sha256:fa10fee7e32213f5c7b0d6428ea92e3a3fdd6d725590238a3f92c0de1c78b9d2"},
+    {file = "coverage-5.3.1-cp37-cp37m-win_amd64.whl", hash = "sha256:ce6f3a147b4b1a8b09aae48517ae91139b1b010c5f36423fa2b866a8b23df879"},
+    {file = "coverage-5.3.1-cp38-cp38-macosx_10_9_x86_64.whl", hash = "sha256:93a280c9eb736a0dcca19296f3c30c720cb41a71b1f9e617f341f0a8e791a69b"},
+    {file = "coverage-5.3.1-cp38-cp38-manylinux1_i686.whl", hash = "sha256:3102bb2c206700a7d28181dbe04d66b30780cde1d1c02c5f3c165cf3d2489497"},
+    {file = "coverage-5.3.1-cp38-cp38-manylinux1_x86_64.whl", hash = "sha256:8ffd4b204d7de77b5dd558cdff986a8274796a1e57813ed005b33fd97e29f059"},
+    {file = "coverage-5.3.1-cp38-cp38-manylinux2010_i686.whl", hash = "sha256:a607ae05b6c96057ba86c811d9c43423f35e03874ffb03fbdcd45e0637e8b631"},
+    {file = "coverage-5.3.1-cp38-cp38-manylinux2010_x86_64.whl", hash = "sha256:3a3c3f8863255f3c31db3889f8055989527173ef6192a283eb6f4db3c579d830"},
+    {file = "coverage-5.3.1-cp38-cp38-win32.whl", hash = "sha256:ff1330e8bc996570221b450e2d539134baa9465f5cb98aff0e0f73f34172e0ae"},
+    {file = "coverage-5.3.1-cp38-cp38-win_amd64.whl", hash = "sha256:3498b27d8236057def41de3585f317abae235dd3a11d33e01736ffedb2ef8606"},
+    {file = "coverage-5.3.1-cp39-cp39-macosx_10_9_x86_64.whl", hash = "sha256:ceb499d2b3d1d7b7ba23abe8bf26df5f06ba8c71127f188333dddcf356b4b63f"},
+    {file = "coverage-5.3.1-cp39-cp39-manylinux1_i686.whl", hash = "sha256:3b14b1da110ea50c8bcbadc3b82c3933974dbeea1832e814aab93ca1163cd4c1"},
+    {file = "coverage-5.3.1-cp39-cp39-manylinux1_x86_64.whl", hash = "sha256:76b2775dda7e78680d688daabcb485dc87cf5e3184a0b3e012e1d40e38527cc8"},
+    {file = "coverage-5.3.1-cp39-cp39-manylinux2010_i686.whl", hash = "sha256:cef06fb382557f66d81d804230c11ab292d94b840b3cb7bf4450778377b592f4"},
+    {file = "coverage-5.3.1-cp39-cp39-manylinux2010_x86_64.whl", hash = "sha256:6f61319e33222591f885c598e3e24f6a4be3533c1d70c19e0dc59e83a71ce27d"},
+    {file = "coverage-5.3.1-cp39-cp39-win32.whl", hash = "sha256:cc6f8246e74dd210d7e2b56c76ceaba1cc52b025cd75dbe96eb48791e0250e98"},
+    {file = "coverage-5.3.1-cp39-cp39-win_amd64.whl", hash = "sha256:2757fa64e11ec12220968f65d086b7a29b6583d16e9a544c889b22ba98555ef1"},
+    {file = "coverage-5.3.1-pp36-none-any.whl", hash = "sha256:723d22d324e7997a651478e9c5a3120a0ecbc9a7e94071f7e1954562a8806cf3"},
+    {file = "coverage-5.3.1-pp37-none-any.whl", hash = "sha256:c89b558f8a9a5a6f2cfc923c304d49f0ce629c3bd85cb442ca258ec20366394c"},
+    {file = "coverage-5.3.1.tar.gz", hash = "sha256:38f16b1317b8dd82df67ed5daa5f5e7c959e46579840d77a67a4ceb9cef0a50b"},
 ]
 distlib = [
     {file = "distlib-0.3.1-py2.py3-none-any.whl", hash = "sha256:8c09de2c67b3e7deef7184574fc060ab8a793e7adbb183d942c389c8b13c52fb"},
@@ -764,8 +774,8 @@ futures = [
     {file = "futures-3.3.0.tar.gz", hash = "sha256:7e033af76a5e35f58e56da7a91e687706faf4e7bdfb2cbc3f2cca6b9bcda9794"},
 ]
 identify = [
-    {file = "identify-1.4.26-py2.py3-none-any.whl", hash = "sha256:150e7c679d6c12e67d18a63808f59068e98576b9ab1fd0ebfcc013c885c5f2e7"},
-    {file = "identify-1.4.26.tar.gz", hash = "sha256:1e9d119ad2aea3af2dec09cf8d4b89af11aa9069ea48240338b53f9dbeedc015"},
+    {file = "identify-1.5.13-py2.py3-none-any.whl", hash = "sha256:9dfb63a2e871b807e3ba62f029813552a24b5289504f5b071dea9b041aee9fe4"},
+    {file = "identify-1.5.13.tar.gz", hash = "sha256:70b638cf4743f33042bebb3b51e25261a0a10e80f978739f17e7fd4837664a66"},
 ]
 idna = [
     {file = "idna-2.10-py2.py3-none-any.whl", hash = "sha256:b97d804b1e9b523befed77c48dacec60e6dcb0b5391d57af6a65a312a90648c0"},
@@ -776,8 +786,8 @@ importlib-metadata = [
     {file = "importlib_metadata-1.7.0.tar.gz", hash = "sha256:90bb658cdbbf6d1735b6341ce708fc7024a3e14e99ffdc5783edea9f9b077f83"},
 ]
 importlib-resources = [
-    {file = "importlib_resources-3.0.0-py2.py3-none-any.whl", hash = "sha256:d028f66b66c0d5732dae86ba4276999855e162a749c92620a38c1d779ed138a7"},
-    {file = "importlib_resources-3.0.0.tar.gz", hash = "sha256:19f745a6eca188b490b1428c8d1d4a0d2368759f32370ea8fb89cad2ab1106c3"},
+    {file = "importlib_resources-3.2.1-py2.py3-none-any.whl", hash = "sha256:e2860cf0c4bc999947228d18be154fa3779c5dde0b882bd2d7b3f4d25e698bd6"},
+    {file = "importlib_resources-3.2.1.tar.gz", hash = "sha256:a9fe213ab6452708ec1b3f4ec6f2881b8ab3645cb4e5efb7fea2bbf05a91db3b"},
 ]
 jsonschema = [
     {file = "jsonschema-3.2.0-py2.py3-none-any.whl", hash = "sha256:4e5b3cf8216f577bee9ce139cbe72eca3ea4f292ec60928ff24758ce626cd163"},
@@ -791,15 +801,16 @@ more-itertools = [
     {file = "more-itertools-5.0.0.tar.gz", hash = "sha256:38a936c0a6d98a38bcc2d03fdaaedaba9f412879461dd2ceff8d37564d6522e4"},
     {file = "more_itertools-5.0.0-py2-none-any.whl", hash = "sha256:c0a5785b1109a6bd7fac76d6837fd1feca158e54e521ccd2ae8bfe393cc9d4fc"},
     {file = "more_itertools-5.0.0-py3-none-any.whl", hash = "sha256:fe7a7cae1ccb57d33952113ff4fa1bc5f879963600ed74918f1236e212ee50b9"},
-    {file = "more-itertools-8.4.0.tar.gz", hash = "sha256:68c70cc7167bdf5c7c9d8f6954a7837089c6a36bf565383919bb595efb8a17e5"},
-    {file = "more_itertools-8.4.0-py3-none-any.whl", hash = "sha256:b78134b2063dd214000685165d81c154522c3ee0a1c0d4d113c80361c234c5a2"},
+    {file = "more-itertools-8.6.0.tar.gz", hash = "sha256:b3a9005928e5bed54076e6e549c792b306fddfe72b2d1d22dd63d42d5d3899cf"},
+    {file = "more_itertools-8.6.0-py3-none-any.whl", hash = "sha256:8e1a2a43b2f2727425f2b5839587ae37093f19153dc26c0927d1048ff6557330"},
 ]
 nodeenv = [
-    {file = "nodeenv-1.4.0-py2.py3-none-any.whl", hash = "sha256:4b0b77afa3ba9b54f4b6396e60b0c83f59eaeb2d63dc3cc7a70f7f4af96c82bc"},
+    {file = "nodeenv-1.5.0-py2.py3-none-any.whl", hash = "sha256:5304d424c529c997bc888453aeaa6362d242b6b4631e90f3d4bf1b290f1c84a9"},
+    {file = "nodeenv-1.5.0.tar.gz", hash = "sha256:ab45090ae383b716c4ef89e690c41ff8c2b257b85b309f01f3654df3d084bd7c"},
 ]
 packaging = [
-    {file = "packaging-20.4-py2.py3-none-any.whl", hash = "sha256:998416ba6962ae7fbd6596850b80e17859a5753ba17c32284f67bfff33784181"},
-    {file = "packaging-20.4.tar.gz", hash = "sha256:4357f74f47b9c12db93624a82154e9b120fa8293699949152b22065d556079f8"},
+    {file = "packaging-20.8-py2.py3-none-any.whl", hash = "sha256:24e0da08660a87484d1602c30bb4902d74816b6985b93de36926f5bc95741858"},
+    {file = "packaging-20.8.tar.gz", hash = "sha256:78598185a7008a470d64526a8059de9aaa449238f280fc9eb6b13ba6c4109093"},
 ]
 pathlib2 = [
     {file = "pathlib2-2.3.5-py2.py3-none-any.whl", hash = "sha256:0ec8205a157c80d7acc301c0b18fbd5d44fe655968f5d947b6ecef5290fc35db"},
@@ -818,23 +829,23 @@ pre-commit = [
     {file = "pre_commit-1.21.0.tar.gz", hash = "sha256:8f48d8637bdae6fa70cc97db9c1dd5aa7c5c8bf71968932a380628c25978b850"},
 ]
 py = [
-    {file = "py-1.9.0-py2.py3-none-any.whl", hash = "sha256:366389d1db726cd2fcfc79732e75410e5fe4d31db13692115529d34069a043c2"},
-    {file = "py-1.9.0.tar.gz", hash = "sha256:9ca6883ce56b4e8da7e79ac18787889fa5206c79dcc67fb065376cd2fe03f342"},
+    {file = "py-1.10.0-py2.py3-none-any.whl", hash = "sha256:3b80836aa6d1feeaa108e046da6423ab8f6ceda6468545ae8d02d9d58d18818a"},
+    {file = "py-1.10.0.tar.gz", hash = "sha256:21b81bda15b66ef5e1a777a21c4dcd9c20ad3efd0b3f817e7a809035269e1bd3"},
 ]
 pyparsing = [
     {file = "pyparsing-2.4.7-py2.py3-none-any.whl", hash = "sha256:ef9d7589ef3c200abe66653d3f1ab1033c3c419ae9b9bdb1240a85b024efc88b"},
     {file = "pyparsing-2.4.7.tar.gz", hash = "sha256:c203ec8783bf771a155b207279b9bccb8dea02d8f0c9e5f8ead507bc3246ecc1"},
 ]
 pyrsistent = [
-    {file = "pyrsistent-0.16.0.tar.gz", hash = "sha256:28669905fe725965daa16184933676547c5bb40a5153055a8dee2a4bd7933ad3"},
+    {file = "pyrsistent-0.16.1.tar.gz", hash = "sha256:aa2ae1c2e496f4d6777f869ea5de7166a8ccb9c2e06ebcf6c7ff1b670c98c5ef"},
 ]
 pytest = [
     {file = "pytest-4.6.11-py2.py3-none-any.whl", hash = "sha256:a00a7d79cbbdfa9d21e7d0298392a8dd4123316bfac545075e6f8f24c94d8c97"},
     {file = "pytest-4.6.11.tar.gz", hash = "sha256:50fa82392f2120cc3ec2ca0a75ee615be4c479e66669789771f1758332be4353"},
 ]
 pytest-cov = [
-    {file = "pytest-cov-2.10.1.tar.gz", hash = "sha256:47bd0ce14056fdd79f93e1713f88fad7bdcc583dcd7783da86ef2f085a0bb88e"},
-    {file = "pytest_cov-2.10.1-py2.py3-none-any.whl", hash = "sha256:45ec2d5182f89a81fc3eb29e3d1ed3113b9e9a873bcddb2a71faaab066110191"},
+    {file = "pytest-cov-2.11.0.tar.gz", hash = "sha256:e90e034cde61dacb1394639a33f449725c591025b182d69752c1dd0bfec639a7"},
+    {file = "pytest_cov-2.11.0-py2.py3-none-any.whl", hash = "sha256:626a8a6ab188656c4f84b67d22436d6c494699d917e567e0048dda6e7f59e028"},
 ]
 pytest-mock = [
     {file = "pytest-mock-2.0.0.tar.gz", hash = "sha256:b35eb281e93aafed138db25c8772b95d3756108b601947f89af503f8c629413f"},
@@ -854,8 +865,8 @@ pyyaml = [
     {file = "PyYAML-5.3.1.tar.gz", hash = "sha256:b8eac752c5e14d3eca0e6dd9199cd627518cb5ec06add0de9d32baeee6fe645d"},
 ]
 requests = [
-    {file = "requests-2.24.0-py2.py3-none-any.whl", hash = "sha256:fe75cc94a9443b9246fc7049224f75604b113c36acb93f87b80ed42c44cbb898"},
-    {file = "requests-2.24.0.tar.gz", hash = "sha256:b3559a131db72c33ee969480840fff4bb6dd111de7dd27c8ee1f820f4f00231b"},
+    {file = "requests-2.25.1-py2.py3-none-any.whl", hash = "sha256:c210084e36a42ae6b9219e00e48287def368a26d03a048ddad7bfee44f75871e"},
+    {file = "requests-2.25.1.tar.gz", hash = "sha256:27973dd4a904a4f13b263a19c866c13b92a39ed1c964655f025f3f8d3d75b804"},
 ]
 scandir = [
     {file = "scandir-1.10.0-cp27-cp27m-win32.whl", hash = "sha256:92c85ac42f41ffdc35b6da57ed991575bdbe69db895507af88b9f499b701c188"},
@@ -879,28 +890,28 @@ six = [
     {file = "six-1.15.0.tar.gz", hash = "sha256:30639c035cdb23534cd4aa2dd52c3bf48f06e5f4a941509c8bafd8ce11080259"},
 ]
 toml = [
-    {file = "toml-0.10.1-py2.py3-none-any.whl", hash = "sha256:bda89d5935c2eac546d648028b9901107a595863cb36bae0c73ac804a9b4ce88"},
-    {file = "toml-0.10.1.tar.gz", hash = "sha256:926b612be1e5ce0634a2ca03470f95169cf16f939018233a670519cb4ac58b0f"},
+    {file = "toml-0.10.2-py2.py3-none-any.whl", hash = "sha256:806143ae5bfb6a3c6e736a764057db0e6a0e05e338b5630894a5f779cabb4f9b"},
+    {file = "toml-0.10.2.tar.gz", hash = "sha256:b3bda1d108d5dd99f4a20d24d9c348e91c4db7ab1b749200bded2f839ccbe68f"},
 ]
 tox = [
-    {file = "tox-3.19.0-py2.py3-none-any.whl", hash = "sha256:3d94b6921a0b6dc90fd8128df83741f30bb41ccd6cd52d131a6a6944ca8f16e6"},
-    {file = "tox-3.19.0.tar.gz", hash = "sha256:17e61a93afe5c49281fb969ab71f7a3f22d7586d1c56f9a74219910f356fe7d3"},
+    {file = "tox-3.21.1-py2.py3-none-any.whl", hash = "sha256:8a28facf65275c84b3bfde102afe24541d2ce02fd83ea07ce906537c3a74ed2d"},
+    {file = "tox-3.21.1.tar.gz", hash = "sha256:31379f2662393034203c5d6b2ebb7b86c67452cfc689784475c1c6e4a3cbc0cd"},
 ]
 typing = [
     {file = "typing-3.7.4.3-py2-none-any.whl", hash = "sha256:283d868f5071ab9ad873e5e52268d611e851c870a2ba354193026f2dfb29d8b5"},
     {file = "typing-3.7.4.3.tar.gz", hash = "sha256:1187fb9c82fd670d10aa07bbb6cfcfe4bdda42d6fab8d5134f04e8c4d0b71cc9"},
 ]
 urllib3 = [
-    {file = "urllib3-1.25.10-py2.py3-none-any.whl", hash = "sha256:e7983572181f5e1522d9c98453462384ee92a0be7fac5f1413a1e35c56cc0461"},
-    {file = "urllib3-1.25.10.tar.gz", hash = "sha256:91056c15fa70756691db97756772bb1eb9678fa585d9184f24534b100dc60f4a"},
+    {file = "urllib3-1.26.2-py2.py3-none-any.whl", hash = "sha256:d8ff90d979214d7b4f8ce956e80f4028fc6860e4431f731ea4a8c08f23f99473"},
+    {file = "urllib3-1.26.2.tar.gz", hash = "sha256:19188f96923873c92ccb987120ec4acaa12f0461fa9ce5d3d0772bc965a39e08"},
 ]
 vendoring = [
-    {file = "vendoring-0.3.2-py2.py3-none-any.whl", hash = "sha256:cbe826d310aae559391ed593f5c8466312451705c822e38380e1d05702e0f6a2"},
-    {file = "vendoring-0.3.2.tar.gz", hash = "sha256:952e851adbd5f3c080f06a19e9dc67c7ea91d85ec482c78758f794bfea4d23f1"},
+    {file = "vendoring-0.3.3-py2.py3-none-any.whl", hash = "sha256:2b91c302116320f903fdb5e60c2b0805d807e2b87425ab0c86624028aa5ffa57"},
+    {file = "vendoring-0.3.3.tar.gz", hash = "sha256:2bbfc0c8da2863f4638c854b91e80c5d0ca57db62fb979d4b0f52088eeab1162"},
 ]
 virtualenv = [
-    {file = "virtualenv-20.0.30-py2.py3-none-any.whl", hash = "sha256:8cd7b2a4850b003a11be2fc213e206419efab41115cc14bca20e69654f2ac08e"},
-    {file = "virtualenv-20.0.30.tar.gz", hash = "sha256:7b54fd606a1b85f83de49ad8d80dbec08e983a2d2f96685045b262ebc7481ee5"},
+    {file = "virtualenv-20.3.1-py2.py3-none-any.whl", hash = "sha256:14b34341e742bdca219e10708198e704e8a7064dd32f474fc16aca68ac53a306"},
+    {file = "virtualenv-20.3.1.tar.gz", hash = "sha256:0c111a2236b191422b37fe8c28b8c828ced39aab4bf5627fa5c331aeffb570d9"},
 ]
 wcwidth = [
     {file = "wcwidth-0.2.5-py2.py3-none-any.whl", hash = "sha256:beb4802a9cebb9144e99086eff703a642a13d6a0052920003a230f3294bbe784"},
diff --git a/poetry/core/_vendor/_pyrsistent_version.py b/poetry/core/_vendor/_pyrsistent_version.py
index 8911e95ca..9513287c9 100644
--- a/poetry/core/_vendor/_pyrsistent_version.py
+++ b/poetry/core/_vendor/_pyrsistent_version.py
@@ -1 +1 @@
-__version__ = '0.16.0'
+__version__ = '0.16.1'
diff --git a/poetry/core/_vendor/attr/__init__.py b/poetry/core/_vendor/attr/__init__.py
index 9ff4d47ff..bf329cad5 100644
--- a/poetry/core/_vendor/attr/__init__.py
+++ b/poetry/core/_vendor/attr/__init__.py
@@ -1,10 +1,12 @@
 from __future__ import absolute_import, division, print_function
 
+import sys
+
 from functools import partial
 
-from . import converters, exceptions, filters, validators
+from . import converters, exceptions, filters, setters, validators
 from ._config import get_run_validators, set_run_validators
-from ._funcs import asdict, assoc, astuple, evolve, has
+from ._funcs import asdict, assoc, astuple, evolve, has, resolve_types
 from ._make import (
     NOTHING,
     Attribute,
@@ -19,7 +21,7 @@
 from ._version_info import VersionInfo
 
 
-__version__ = "19.3.0"
+__version__ = "20.3.0"
 __version_info__ = VersionInfo._from_version_string(__version__)
 
 __title__ = "attrs"
@@ -39,7 +41,6 @@
 ib = attr = attrib
 dataclass = partial(attrs, auto_attribs=True)  # happy Easter ;)
 
-
 __all__ = [
     "Attribute",
     "Factory",
@@ -61,8 +62,15 @@
     "has",
     "ib",
     "make_class",
+    "resolve_types",
     "s",
     "set_run_validators",
+    "setters",
     "validate",
     "validators",
 ]
+
+if sys.version_info[:2] >= (3, 6):
+    from ._next_gen import define, field, frozen, mutable
+
+    __all__.extend((define, field, frozen, mutable))
diff --git a/poetry/core/_vendor/attr/_compat.py b/poetry/core/_vendor/attr/_compat.py
index a915db8eb..b0ead6e1c 100644
--- a/poetry/core/_vendor/attr/_compat.py
+++ b/poetry/core/_vendor/attr/_compat.py
@@ -19,9 +19,10 @@
 
 
 if PY2:
-    from UserDict import IterableUserDict
     from collections import Mapping, Sequence
 
+    from UserDict import IterableUserDict
+
     # We 'bundle' isclass instead of using inspect as importing inspect is
     # fairly expensive (order of 10-15 ms for a modern machine in 2016)
     def isclass(klass):
@@ -90,7 +91,7 @@ def metadata_proxy(d):
         res.data.update(d)  # We blocked update, so we have to do it like this.
         return res
 
-    def just_warn(*args, **kw):  # pragma: nocover
+    def just_warn(*args, **kw):  # pragma: no cover
         """
         We only warn on Python 3 because we are not aware of any concrete
         consequences of not setting the cell on Python 2.
@@ -131,7 +132,7 @@ def make_set_closure_cell():
     """
     # pypy makes this easy. (It also supports the logic below, but
     # why not do the easy/fast thing?)
-    if PYPY:  # pragma: no cover
+    if PYPY:
 
         def set_closure_cell(cell, value):
             cell.__setstate__((value,))
diff --git a/poetry/core/_vendor/attr/_funcs.py b/poetry/core/_vendor/attr/_funcs.py
index c077e4284..e6c930cbd 100644
--- a/poetry/core/_vendor/attr/_funcs.py
+++ b/poetry/core/_vendor/attr/_funcs.py
@@ -13,6 +13,7 @@ def asdict(
     filter=None,
     dict_factory=dict,
     retain_collection_types=False,
+    value_serializer=None,
 ):
     """
     Return the ``attrs`` attribute values of *inst* as a dict.
@@ -32,6 +33,10 @@ def asdict(
     :param bool retain_collection_types: Do not convert to ``list`` when
         encountering an attribute whose type is ``tuple`` or ``set``.  Only
         meaningful if ``recurse`` is ``True``.
+    :param Optional[callable] value_serializer: A hook that is called for every
+        attribute or dict key/value.  It receives the current instance, field
+        and value and must return the (updated) value.  The hook is run *after*
+        the optional *filter* has been applied.
 
     :rtype: return type of *dict_factory*
 
@@ -40,6 +45,7 @@ def asdict(
 
     ..  versionadded:: 16.0.0 *dict_factory*
     ..  versionadded:: 16.1.0 *retain_collection_types*
+    ..  versionadded:: 20.3.0 *value_serializer*
     """
     attrs = fields(inst.__class__)
     rv = dict_factory()
@@ -47,17 +53,30 @@ def asdict(
         v = getattr(inst, a.name)
         if filter is not None and not filter(a, v):
             continue
+
+        if value_serializer is not None:
+            v = value_serializer(inst, a, v)
+
         if recurse is True:
             if has(v.__class__):
                 rv[a.name] = asdict(
-                    v, True, filter, dict_factory, retain_collection_types
+                    v,
+                    True,
+                    filter,
+                    dict_factory,
+                    retain_collection_types,
+                    value_serializer,
                 )
-            elif isinstance(v, (tuple, list, set)):
+            elif isinstance(v, (tuple, list, set, frozenset)):
                 cf = v.__class__ if retain_collection_types is True else list
                 rv[a.name] = cf(
                     [
                         _asdict_anything(
-                            i, filter, dict_factory, retain_collection_types
+                            i,
+                            filter,
+                            dict_factory,
+                            retain_collection_types,
+                            value_serializer,
                         )
                         for i in v
                     ]
@@ -67,10 +86,18 @@ def asdict(
                 rv[a.name] = df(
                     (
                         _asdict_anything(
-                            kk, filter, df, retain_collection_types
+                            kk,
+                            filter,
+                            df,
+                            retain_collection_types,
+                            value_serializer,
                         ),
                         _asdict_anything(
-                            vv, filter, df, retain_collection_types
+                            vv,
+                            filter,
+                            df,
+                            retain_collection_types,
+                            value_serializer,
                         ),
                     )
                     for kk, vv in iteritems(v)
@@ -82,19 +109,36 @@ def asdict(
     return rv
 
 
-def _asdict_anything(val, filter, dict_factory, retain_collection_types):
+def _asdict_anything(
+    val,
+    filter,
+    dict_factory,
+    retain_collection_types,
+    value_serializer,
+):
     """
     ``asdict`` only works on attrs instances, this works on anything.
     """
     if getattr(val.__class__, "__attrs_attrs__", None) is not None:
         # Attrs class.
-        rv = asdict(val, True, filter, dict_factory, retain_collection_types)
-    elif isinstance(val, (tuple, list, set)):
+        rv = asdict(
+            val,
+            True,
+            filter,
+            dict_factory,
+            retain_collection_types,
+            value_serializer,
+        )
+    elif isinstance(val, (tuple, list, set, frozenset)):
         cf = val.__class__ if retain_collection_types is True else list
         rv = cf(
             [
                 _asdict_anything(
-                    i, filter, dict_factory, retain_collection_types
+                    i,
+                    filter,
+                    dict_factory,
+                    retain_collection_types,
+                    value_serializer,
                 )
                 for i in val
             ]
@@ -103,13 +147,20 @@ def _asdict_anything(val, filter, dict_factory, retain_collection_types):
         df = dict_factory
         rv = df(
             (
-                _asdict_anything(kk, filter, df, retain_collection_types),
-                _asdict_anything(vv, filter, df, retain_collection_types),
+                _asdict_anything(
+                    kk, filter, df, retain_collection_types, value_serializer
+                ),
+                _asdict_anything(
+                    vv, filter, df, retain_collection_types, value_serializer
+                ),
             )
             for kk, vv in iteritems(val)
         )
     else:
         rv = val
+        if value_serializer is not None:
+            rv = value_serializer(None, None, rv)
+
     return rv
 
 
@@ -164,7 +215,7 @@ def astuple(
                         retain_collection_types=retain,
                     )
                 )
-            elif isinstance(v, (tuple, list, set)):
+            elif isinstance(v, (tuple, list, set, frozenset)):
                 cf = v.__class__ if retain is True else list
                 rv.append(
                     cf(
@@ -209,6 +260,7 @@ def astuple(
                 rv.append(v)
         else:
             rv.append(v)
+
     return rv if tuple_factory is list else tuple_factory(rv)
 
 
@@ -287,4 +339,52 @@ def evolve(inst, **changes):
         init_name = attr_name if attr_name[0] != "_" else attr_name[1:]
         if init_name not in changes:
             changes[init_name] = getattr(inst, attr_name)
+
     return cls(**changes)
+
+
+def resolve_types(cls, globalns=None, localns=None):
+    """
+    Resolve any strings and forward annotations in type annotations.
+
+    This is only required if you need concrete types in `Attribute`'s *type*
+    field. In other words, you don't need to resolve your types if you only
+    use them for static type checking.
+
+    With no arguments, names will be looked up in the module in which the class
+    was created. If this is not what you want, e.g. if the name only exists
+    inside a method, you may pass *globalns* or *localns* to specify other
+    dictionaries in which to look up these names. See the docs of
+    `typing.get_type_hints` for more details.
+
+    :param type cls: Class to resolve.
+    :param Optional[dict] globalns: Dictionary containing global variables.
+    :param Optional[dict] localns: Dictionary containing local variables.
+
+    :raise TypeError: If *cls* is not a class.
+    :raise attr.exceptions.NotAnAttrsClassError: If *cls* is not an ``attrs``
+        class.
+    :raise NameError: If types cannot be resolved because of missing variables.
+
+    :returns: *cls* so you can use this function also as a class decorator.
+        Please note that you have to apply it **after** `attr.s`. That means
+        the decorator has to come in the line **before** `attr.s`.
+
+    ..  versionadded:: 20.1.0
+    """
+    try:
+        # Since calling get_type_hints is expensive we cache whether we've
+        # done it already.
+        cls.__attrs_types_resolved__
+    except AttributeError:
+        import typing
+
+        hints = typing.get_type_hints(cls, globalns=globalns, localns=localns)
+        for field in fields(cls):
+            if field.name in hints:
+                # Since fields have been frozen we must work around it.
+                _obj_setattr(field, "type", hints[field.name])
+        cls.__attrs_types_resolved__ = True
+
+    # Return the class so you can use it as a decorator too.
+    return cls
diff --git a/poetry/core/_vendor/attr/_make.py b/poetry/core/_vendor/attr/_make.py
index 46f9c54ec..49484f935 100644
--- a/poetry/core/_vendor/attr/_make.py
+++ b/poetry/core/_vendor/attr/_make.py
@@ -9,9 +9,10 @@
 
 from operator import itemgetter
 
-from . import _config
+from . import _config, setters
 from ._compat import (
     PY2,
+    PYPY,
     isclass,
     iteritems,
     metadata_proxy,
@@ -29,7 +30,7 @@
 
 # This is used at least twice, so cache it here.
 _obj_setattr = object.__setattr__
-_init_converter_pat = "__attr_converter_{}"
+_init_converter_pat = "__attr_converter_%s"
 _init_factory_pat = "__attr_factory_{}"
 _tuple_property_pat = (
     "    {attr_name} = _attrs_property(_attrs_itemgetter({index}))"
@@ -70,6 +71,31 @@ def __repr__(self):
 """
 
 
+class _CacheHashWrapper(int):
+    """
+    An integer subclass that pickles / copies as None
+
+    This is used for non-slots classes with ``cache_hash=True``, to avoid
+    serializing a potentially (even likely) invalid hash value. Since ``None``
+    is the default value for uncalculated hashes, whenever this is copied,
+    the copy's value for the hash should automatically reset.
+
+    See GH #613 for more details.
+    """
+
+    if PY2:
+        # For some reason `type(None)` isn't callable in Python 2, but we don't
+        # actually need a constructor for None objects, we just need any
+        # available function that returns None.
+        def __reduce__(self, _none_constructor=getattr, _args=(0, "", None)):
+            return _none_constructor, _args
+
+    else:
+
+        def __reduce__(self, _none_constructor=type(None), _args=()):
+            return _none_constructor, _args
+
+
 def attrib(
     default=NOTHING,
     validator=None,
@@ -84,6 +110,7 @@ def attrib(
     kw_only=False,
     eq=None,
     order=None,
+    on_setattr=None,
 ):
     """
     Create a new attribute on a class.
@@ -101,7 +128,7 @@ def attrib(
         used to construct a new value (useful for mutable data types like lists
         or dicts).
 
-        If a default is not set (or set manually to ``attr.NOTHING``), a value
+        If a default is not set (or set manually to `attr.NOTHING`), a value
         *must* be supplied when instantiating; otherwise a `TypeError`
         will be raised.
 
@@ -110,7 +137,7 @@ def attrib(
     :type default: Any value
 
     :param callable factory: Syntactic sugar for
-        ``default=attr.Factory(callable)``.
+        ``default=attr.Factory(factory)``.
 
     :param validator: `callable` that is called by ``attrs``-generated
         ``__init__`` methods after the instance has been initialized.  They
@@ -120,7 +147,7 @@ def attrib(
         The return value is *not* inspected so the validator has to throw an
         exception itself.
 
-        If a ``list`` is passed, its items are treated as validators and must
+        If a `list` is passed, its items are treated as validators and must
         all pass.
 
         Validators can be globally disabled and re-enabled using
@@ -128,7 +155,7 @@ def attrib(
 
         The validator can also be set using decorator notation as shown below.
 
-    :type validator: ``callable`` or a ``list`` of ``callable``\\ s.
+    :type validator: `callable` or a `list` of `callable`\\ s.
 
     :param repr: Include this attribute in the generated ``__repr__``
         method. If ``True``, include the attribute; if ``False``, omit it. By
@@ -137,7 +164,7 @@ def attrib(
         value and returns a string. Note that the resulting string is used
         as-is, i.e. it will be used directly *instead* of calling ``repr()``
         (the default).
-    :type repr: a ``bool`` or a ``callable`` to use a custom function.
+    :type repr: a `bool` or a `callable` to use a custom function.
     :param bool eq: If ``True`` (default), include this attribute in the
         generated ``__eq__`` and ``__ne__`` methods that check two instances
         for equality.
@@ -145,17 +172,16 @@ def attrib(
         generated ``__lt__``, ``__le__``, ``__gt__`` and ``__ge__`` methods.
     :param bool cmp: Setting to ``True`` is equivalent to setting ``eq=True,
         order=True``. Deprecated in favor of *eq* and *order*.
-    :param hash: Include this attribute in the generated ``__hash__``
-        method.  If ``None`` (default), mirror *eq*'s value.  This is the
-        correct behavior according the Python spec.  Setting this value to
-        anything else than ``None`` is *discouraged*.
-    :type hash: ``bool`` or ``None``
+    :param Optional[bool] hash: Include this attribute in the generated
+        ``__hash__`` method.  If ``None`` (default), mirror *eq*'s value.  This
+        is the correct behavior according the Python spec.  Setting this value
+        to anything else than ``None`` is *discouraged*.
     :param bool init: Include this attribute in the generated ``__init__``
         method.  It is possible to set this to ``False`` and set a default
         value.  In that case this attributed is unconditionally initialized
         with the specified default value or factory.
     :param callable converter: `callable` that is called by
-        ``attrs``-generated ``__init__`` methods to converter attribute's value
+        ``attrs``-generated ``__init__`` methods to convert attribute's value
         to the desired format.  It is given the passed-in value, and the
         returned value will be used as the new value of the attribute.  The
         value is converted before being passed to the validator, if any.
@@ -174,6 +200,12 @@ def attrib(
     :param kw_only: Make this attribute keyword-only (Python 3+)
         in the generated ``__init__`` (if ``init`` is ``False``, this
         parameter is ignored).
+    :param on_setattr: Allows to overwrite the *on_setattr* setting from
+        `attr.s`. If left `None`, the *on_setattr* value from `attr.s` is used.
+        Set to `attr.setters.NO_OP` to run **no** `setattr` hooks for this
+        attribute -- regardless of the setting in `attr.s`.
+    :type on_setattr: `callable`, or a list of callables, or `None`, or
+        `attr.setters.NO_OP`
 
     .. versionadded:: 15.2.0 *convert*
     .. versionadded:: 16.3.0 *metadata*
@@ -191,8 +223,10 @@ def attrib(
     .. versionchanged:: 19.2.0 *repr* also accepts a custom callable.
     .. deprecated:: 19.2.0 *cmp* Removal on or after 2021-06-01.
     .. versionadded:: 19.2.0 *eq* and *order*
+    .. versionadded:: 20.1.0 *on_setattr*
+    .. versionchanged:: 20.3.0 *kw_only* backported to Python 2
     """
-    eq, order = _determine_eq_order(cmp, eq, order)
+    eq, order = _determine_eq_order(cmp, eq, order, True)
 
     if hash is not None and hash is not True and hash is not False:
         raise TypeError(
@@ -212,6 +246,16 @@ def attrib(
     if metadata is None:
         metadata = {}
 
+    # Apply syntactic sugar by auto-wrapping.
+    if isinstance(on_setattr, (list, tuple)):
+        on_setattr = setters.pipe(*on_setattr)
+
+    if validator and isinstance(validator, (list, tuple)):
+        validator = and_(*validator)
+
+    if converter and isinstance(converter, (list, tuple)):
+        converter = pipe(*converter)
+
     return _CountingAttr(
         default=default,
         validator=validator,
@@ -225,6 +269,7 @@ def attrib(
         kw_only=kw_only,
         eq=eq,
         order=order,
+        on_setattr=on_setattr,
     )
 
 
@@ -282,20 +327,32 @@ def _is_class_var(annot):
     return str(annot).startswith(_classvar_prefixes)
 
 
-def _get_annotations(cls):
+def _has_own_attribute(cls, attrib_name):
     """
-    Get annotations for *cls*.
+    Check whether *cls* defines *attrib_name* (and doesn't just inherit it).
+
+    Requires Python 3.
     """
-    anns = getattr(cls, "__annotations__", None)
-    if anns is None:
-        return {}
+    attr = getattr(cls, attrib_name, _sentinel)
+    if attr is _sentinel:
+        return False
 
-    # Verify that the annotations aren't merely inherited.
     for base_cls in cls.__mro__[1:]:
-        if anns is getattr(base_cls, "__annotations__", None):
-            return {}
+        a = getattr(base_cls, attrib_name, None)
+        if attr is a:
+            return False
 
-    return anns
+    return True
+
+
+def _get_annotations(cls):
+    """
+    Get annotations for *cls*.
+    """
+    if _has_own_attribute(cls, "__annotations__"):
+        return cls.__annotations__
+
+    return {}
 
 
 def _counter_getter(e):
@@ -305,12 +362,76 @@ def _counter_getter(e):
     return e[1].counter
 
 
-def _transform_attrs(cls, these, auto_attribs, kw_only):
+def _collect_base_attrs(cls, taken_attr_names):
+    """
+    Collect attr.ibs from base classes of *cls*, except *taken_attr_names*.
+    """
+    base_attrs = []
+    base_attr_map = {}  # A dictionary of base attrs to their classes.
+
+    # Traverse the MRO and collect attributes.
+    for base_cls in reversed(cls.__mro__[1:-1]):
+        for a in getattr(base_cls, "__attrs_attrs__", []):
+            if a.inherited or a.name in taken_attr_names:
+                continue
+
+            a = a.evolve(inherited=True)
+            base_attrs.append(a)
+            base_attr_map[a.name] = base_cls
+
+    # For each name, only keep the freshest definition i.e. the furthest at the
+    # back.  base_attr_map is fine because it gets overwritten with every new
+    # instance.
+    filtered = []
+    seen = set()
+    for a in reversed(base_attrs):
+        if a.name in seen:
+            continue
+        filtered.insert(0, a)
+        seen.add(a.name)
+
+    return filtered, base_attr_map
+
+
+def _collect_base_attrs_broken(cls, taken_attr_names):
+    """
+    Collect attr.ibs from base classes of *cls*, except *taken_attr_names*.
+
+    N.B. *taken_attr_names* will be mutated.
+
+    Adhere to the old incorrect behavior.
+
+    Notably it collects from the front and considers inherited attributes which
+    leads to the buggy behavior reported in #428.
+    """
+    base_attrs = []
+    base_attr_map = {}  # A dictionary of base attrs to their classes.
+
+    # Traverse the MRO and collect attributes.
+    for base_cls in cls.__mro__[1:-1]:
+        for a in getattr(base_cls, "__attrs_attrs__", []):
+            if a.name in taken_attr_names:
+                continue
+
+            a = a.evolve(inherited=True)
+            taken_attr_names.add(a.name)
+            base_attrs.append(a)
+            base_attr_map[a.name] = base_cls
+
+    return base_attrs, base_attr_map
+
+
+def _transform_attrs(
+    cls, these, auto_attribs, kw_only, collect_by_mro, field_transformer
+):
     """
     Transform all `_CountingAttr`s on a class into `Attribute`s.
 
     If *these* is passed, use that and don't look for them on the class.
 
+    *collect_by_mro* is True, collect them in the correct MRO order, otherwise
+    use the old -- incorrect -- order.  See #428.
+
     Return an `_Attributes`.
     """
     cd = cls.__dict__
@@ -334,6 +455,7 @@ def _transform_attrs(cls, these, auto_attribs, kw_only):
                 continue
             annot_names.add(attr_name)
             a = cd.get(attr_name, NOTHING)
+
             if not isinstance(a, _CountingAttr):
                 if a is NOTHING:
                     a = attrib()
@@ -367,30 +489,22 @@ def _transform_attrs(cls, these, auto_attribs, kw_only):
         for attr_name, ca in ca_list
     ]
 
-    base_attrs = []
-    base_attr_map = {}  # A dictionary of base attrs to their classes.
-    taken_attr_names = {a.name: a for a in own_attrs}
-
-    # Traverse the MRO and collect attributes.
-    for base_cls in cls.__mro__[1:-1]:
-        sub_attrs = getattr(base_cls, "__attrs_attrs__", None)
-        if sub_attrs is not None:
-            for a in sub_attrs:
-                prev_a = taken_attr_names.get(a.name)
-                # Only add an attribute if it hasn't been defined before.  This
-                # allows for overwriting attribute definitions by subclassing.
-                if prev_a is None:
-                    base_attrs.append(a)
-                    taken_attr_names[a.name] = a
-                    base_attr_map[a.name] = base_cls
+    if collect_by_mro:
+        base_attrs, base_attr_map = _collect_base_attrs(
+            cls, {a.name for a in own_attrs}
+        )
+    else:
+        base_attrs, base_attr_map = _collect_base_attrs_broken(
+            cls, {a.name for a in own_attrs}
+        )
 
     attr_names = [a.name for a in base_attrs + own_attrs]
 
     AttrsClass = _make_attr_tuple_class(cls.__name__, attr_names)
 
     if kw_only:
-        own_attrs = [a._assoc(kw_only=True) for a in own_attrs]
-        base_attrs = [a._assoc(kw_only=True) for a in base_attrs]
+        own_attrs = [a.evolve(kw_only=True) for a in own_attrs]
+        base_attrs = [a.evolve(kw_only=True) for a in base_attrs]
 
     attrs = AttrsClass(base_attrs + own_attrs)
 
@@ -409,14 +523,34 @@ def _transform_attrs(cls, these, auto_attribs, kw_only):
         if had_default is False and a.default is not NOTHING:
             had_default = True
 
+    if field_transformer is not None:
+        attrs = field_transformer(cls, attrs)
     return _Attributes((attrs, base_attrs, base_attr_map))
 
 
-def _frozen_setattrs(self, name, value):
-    """
-    Attached to frozen classes as __setattr__.
-    """
-    raise FrozenInstanceError()
+if PYPY:
+
+    def _frozen_setattrs(self, name, value):
+        """
+        Attached to frozen classes as __setattr__.
+        """
+        if isinstance(self, BaseException) and name in (
+            "__cause__",
+            "__context__",
+        ):
+            BaseException.__setattr__(self, name, value)
+            return
+
+        raise FrozenInstanceError()
+
+
+else:
+
+    def _frozen_setattrs(self, name, value):
+        """
+        Attached to frozen classes as __setattr__.
+        """
+        raise FrozenInstanceError()
 
 
 def _frozen_delattrs(self, name):
@@ -432,19 +566,22 @@ class _ClassBuilder(object):
     """
 
     __slots__ = (
-        "_cls",
-        "_cls_dict",
+        "_attr_names",
         "_attrs",
+        "_base_attr_map",
         "_base_names",
-        "_attr_names",
-        "_slots",
-        "_frozen",
-        "_weakref_slot",
         "_cache_hash",
-        "_has_post_init",
+        "_cls",
+        "_cls_dict",
         "_delete_attribs",
-        "_base_attr_map",
+        "_frozen",
+        "_has_post_init",
         "_is_exc",
+        "_on_setattr",
+        "_slots",
+        "_weakref_slot",
+        "_has_own_setattr",
+        "_has_custom_setattr",
     )
 
     def __init__(
@@ -454,13 +591,23 @@ def __init__(
         slots,
         frozen,
         weakref_slot,
+        getstate_setstate,
         auto_attribs,
         kw_only,
         cache_hash,
         is_exc,
+        collect_by_mro,
+        on_setattr,
+        has_custom_setattr,
+        field_transformer,
     ):
         attrs, base_attrs, base_map = _transform_attrs(
-            cls, these, auto_attribs, kw_only
+            cls,
+            these,
+            auto_attribs,
+            kw_only,
+            collect_by_mro,
+            field_transformer,
         )
 
         self._cls = cls
@@ -470,12 +617,16 @@ def __init__(
         self._base_attr_map = base_map
         self._attr_names = tuple(a.name for a in attrs)
         self._slots = slots
-        self._frozen = frozen or _has_frozen_base_class(cls)
+        self._frozen = frozen
         self._weakref_slot = weakref_slot
         self._cache_hash = cache_hash
         self._has_post_init = bool(getattr(cls, "__attrs_post_init__", False))
         self._delete_attribs = not bool(these)
         self._is_exc = is_exc
+        self._on_setattr = on_setattr
+
+        self._has_custom_setattr = has_custom_setattr
+        self._has_own_setattr = False
 
         self._cls_dict["__attrs_attrs__"] = self._attrs
 
@@ -483,6 +634,14 @@ def __init__(
             self._cls_dict["__setattr__"] = _frozen_setattrs
             self._cls_dict["__delattr__"] = _frozen_delattrs
 
+            self._has_own_setattr = True
+
+        if getstate_setstate:
+            (
+                self._cls_dict["__getstate__"],
+                self._cls_dict["__setstate__"],
+            ) = self._make_getstate_setstate()
+
     def __repr__(self):
         return "<_ClassBuilder(cls={cls})>".format(cls=self._cls.__name__)
 
@@ -523,25 +682,15 @@ def _patch_original_class(self):
         for name, value in self._cls_dict.items():
             setattr(cls, name, value)
 
-        # Attach __setstate__. This is necessary to clear the hash code
-        # cache on deserialization. See issue
-        # https://github.com/python-attrs/attrs/issues/482 .
-        # Note that this code only handles setstate for dict classes.
-        # For slotted classes, see similar code in _create_slots_class .
-        if self._cache_hash:
-            existing_set_state_method = getattr(cls, "__setstate__", None)
-            if existing_set_state_method:
-                raise NotImplementedError(
-                    "Currently you cannot use hash caching if "
-                    "you specify your own __setstate__ method."
-                    "See https://github.com/python-attrs/attrs/issues/494 ."
-                )
-
-            def cache_hash_set_state(chss_self, _):
-                # clear hash code cache
-                setattr(chss_self, _hash_cache_field, None)
+        # If we've inherited an attrs __setattr__ and don't write our own,
+        # reset it to object's.
+        if not self._has_own_setattr and getattr(
+            cls, "__attrs_own_setattr__", False
+        ):
+            cls.__attrs_own_setattr__ = False
 
-            setattr(cls, "__setstate__", cache_hash_set_state)
+            if not self._has_custom_setattr:
+                cls.__setattr__ = object.__setattr__
 
         return cls
 
@@ -556,11 +705,27 @@ def _create_slots_class(self):
             if k not in tuple(self._attr_names) + ("__dict__", "__weakref__")
         }
 
-        weakref_inherited = False
+        # If our class doesn't have its own implementation of __setattr__
+        # (either from the user or by us), check the bases, if one of them has
+        # an attrs-made __setattr__, that needs to be reset. We don't walk the
+        # MRO because we only care about our immediate base classes.
+        # XXX: This can be confused by subclassing a slotted attrs class with
+        # XXX: a non-attrs class and subclass the resulting class with an attrs
+        # XXX: class.  See `test_slotted_confused` for details.  For now that's
+        # XXX: OK with us.
+        if not self._has_own_setattr:
+            cd["__attrs_own_setattr__"] = False
+
+            if not self._has_custom_setattr:
+                for base_cls in self._cls.__bases__:
+                    if base_cls.__dict__.get("__attrs_own_setattr__", False):
+                        cd["__setattr__"] = object.__setattr__
+                        break
 
         # Traverse the MRO to check for an existing __weakref__.
+        weakref_inherited = False
         for base_cls in self._cls.__mro__[1:-1]:
-            if "__weakref__" in getattr(base_cls, "__dict__", ()):
+            if base_cls.__dict__.get("__weakref__", None) is not None:
                 weakref_inherited = True
                 break
 
@@ -574,7 +739,7 @@ def _create_slots_class(self):
             names += ("__weakref__",)
 
         # We only add the names of attributes that aren't inherited.
-        # Settings __slots__ to inherited attributes wastes memory.
+        # Setting __slots__ to inherited attributes wastes memory.
         slot_names = [name for name in names if name not in base_names]
         if self._cache_hash:
             slot_names.append(_hash_cache_field)
@@ -584,38 +749,6 @@ def _create_slots_class(self):
         if qualname is not None:
             cd["__qualname__"] = qualname
 
-        # __weakref__ is not writable.
-        state_attr_names = tuple(
-            an for an in self._attr_names if an != "__weakref__"
-        )
-
-        def slots_getstate(self):
-            """
-            Automatically created by attrs.
-            """
-            return tuple(getattr(self, name) for name in state_attr_names)
-
-        hash_caching_enabled = self._cache_hash
-
-        def slots_setstate(self, state):
-            """
-            Automatically created by attrs.
-            """
-            __bound_setattr = _obj_setattr.__get__(self, Attribute)
-            for name, value in zip(state_attr_names, state):
-                __bound_setattr(name, value)
-            # Clearing the hash code cache on deserialization is needed
-            # because hash codes can change from run to run. See issue
-            # https://github.com/python-attrs/attrs/issues/482 .
-            # Note that this code only handles setstate for slotted classes.
-            # For dict classes, see similar code in _patch_original_class .
-            if hash_caching_enabled:
-                __bound_setattr(_hash_cache_field, None)
-
-        # slots and frozen require __getstate__/__setstate__ to work
-        cd["__getstate__"] = slots_getstate
-        cd["__setstate__"] = slots_setstate
-
         # Create new class based on old class and our methods.
         cls = type(self._cls)(self._cls.__name__, self._cls.__bases__, cd)
 
@@ -636,8 +769,13 @@ def slots_setstate(self, state):
             if not closure_cells:  # Catch None or the empty list.
                 continue
             for cell in closure_cells:
-                if cell.cell_contents is self._cls:
-                    set_closure_cell(cell, cls)
+                try:
+                    match = cell.cell_contents is self._cls
+                except ValueError:  # ValueError: Cell is empty
+                    pass
+                else:
+                    if match:
+                        set_closure_cell(cell, cls)
 
         return cls
 
@@ -660,6 +798,40 @@ def __str__(self):
         self._cls_dict["__str__"] = self._add_method_dunders(__str__)
         return self
 
+    def _make_getstate_setstate(self):
+        """
+        Create custom __setstate__ and __getstate__ methods.
+        """
+        # __weakref__ is not writable.
+        state_attr_names = tuple(
+            an for an in self._attr_names if an != "__weakref__"
+        )
+
+        def slots_getstate(self):
+            """
+            Automatically created by attrs.
+            """
+            return tuple(getattr(self, name) for name in state_attr_names)
+
+        hash_caching_enabled = self._cache_hash
+
+        def slots_setstate(self, state):
+            """
+            Automatically created by attrs.
+            """
+            __bound_setattr = _obj_setattr.__get__(self, Attribute)
+            for name, value in zip(state_attr_names, state):
+                __bound_setattr(name, value)
+
+            # The hash code cache is not included when the object is
+            # serialized, but it still needs to be initialized to None to
+            # indicate that the first call to __hash__ should be a cache
+            # miss.
+            if hash_caching_enabled:
+                __bound_setattr(_hash_cache_field, None)
+
+        return slots_getstate, slots_setstate
+
     def make_unhashable(self):
         self._cls_dict["__hash__"] = None
         return self
@@ -687,6 +859,8 @@ def add_init(self):
                 self._cache_hash,
                 self._base_attr_map,
                 self._is_exc,
+                self._on_setattr is not None
+                and self._on_setattr is not setters.NO_OP,
             )
         )
 
@@ -695,10 +869,10 @@ def add_init(self):
     def add_eq(self):
         cd = self._cls_dict
 
-        cd["__eq__"], cd["__ne__"] = (
-            self._add_method_dunders(meth)
-            for meth in _make_eq(self._cls, self._attrs)
+        cd["__eq__"] = self._add_method_dunders(
+            _make_eq(self._cls, self._attrs)
         )
+        cd["__ne__"] = self._add_method_dunders(_make_ne())
 
         return self
 
@@ -712,6 +886,42 @@ def add_order(self):
 
         return self
 
+    def add_setattr(self):
+        if self._frozen:
+            return self
+
+        sa_attrs = {}
+        for a in self._attrs:
+            on_setattr = a.on_setattr or self._on_setattr
+            if on_setattr and on_setattr is not setters.NO_OP:
+                sa_attrs[a.name] = a, on_setattr
+
+        if not sa_attrs:
+            return self
+
+        if self._has_custom_setattr:
+            # We need to write a __setattr__ but there already is one!
+            raise ValueError(
+                "Can't combine custom __setattr__ with on_setattr hooks."
+            )
+
+        # docstring comes from _add_method_dunders
+        def __setattr__(self, name, val):
+            try:
+                a, hook = sa_attrs[name]
+            except KeyError:
+                nval = val
+            else:
+                nval = hook(self, a, val)
+
+            _obj_setattr(self, name, nval)
+
+        self._cls_dict["__attrs_own_setattr__"] = True
+        self._cls_dict["__setattr__"] = self._add_method_dunders(__setattr__)
+        self._has_own_setattr = True
+
+        return self
+
     def _add_method_dunders(self, method):
         """
         Add __module__ and __qualname__ to a *method* if possible.
@@ -728,6 +938,13 @@ def _add_method_dunders(self, method):
         except AttributeError:
             pass
 
+        try:
+            method.__doc__ = "Method generated by attrs for class %s." % (
+                self._cls.__qualname__,
+            )
+        except AttributeError:
+            pass
+
         return method
 
 
@@ -737,10 +954,10 @@ def _add_method_dunders(self, method):
 )
 
 
-def _determine_eq_order(cmp, eq, order):
+def _determine_eq_order(cmp, eq, order, default_eq):
     """
     Validate the combination of *cmp*, *eq*, and *order*. Derive the effective
-    values of eq and order.
+    values of eq and order.  If *eq* is None, set it to *default_eq*.
     """
     if cmp is not None and any((eq is not None, order is not None)):
         raise ValueError("Don't mix `cmp` with `eq' and `order`.")
@@ -751,9 +968,10 @@ def _determine_eq_order(cmp, eq, order):
 
         return cmp, cmp
 
-    # If left None, equality is on and ordering mirrors equality.
+    # If left None, equality is set to the specified default and ordering
+    # mirrors equality.
     if eq is None:
-        eq = True
+        eq = default_eq
 
     if order is None:
         order = eq
@@ -764,14 +982,42 @@ def _determine_eq_order(cmp, eq, order):
     return eq, order
 
 
+def _determine_whether_to_implement(
+    cls, flag, auto_detect, dunders, default=True
+):
+    """
+    Check whether we should implement a set of methods for *cls*.
+
+    *flag* is the argument passed into @attr.s like 'init', *auto_detect* the
+    same as passed into @attr.s and *dunders* is a tuple of attribute names
+    whose presence signal that the user has implemented it themselves.
+
+    Return *default* if no reason for either for or against is found.
+
+    auto_detect must be False on Python 2.
+    """
+    if flag is True or flag is False:
+        return flag
+
+    if flag is None and auto_detect is False:
+        return default
+
+    # Logically, flag is None and auto_detect is True here.
+    for dunder in dunders:
+        if _has_own_attribute(cls, dunder):
+            return False
+
+    return default
+
+
 def attrs(
     maybe_cls=None,
     these=None,
     repr_ns=None,
-    repr=True,
+    repr=None,
     cmp=None,
     hash=None,
-    init=True,
+    init=None,
     slots=False,
     frozen=False,
     weakref_slot=True,
@@ -782,6 +1028,11 @@ def attrs(
     auto_exc=False,
     eq=None,
     order=None,
+    auto_detect=False,
+    collect_by_mro=False,
+    getstate_setstate=None,
+    on_setattr=None,
+    field_transformer=None,
 ):
     r"""
     A class decorator that adds `dunder
@@ -806,28 +1057,52 @@ def attrs(
     :param str repr_ns: When using nested classes, there's no way in Python 2
         to automatically detect that.  Therefore it's possible to set the
         namespace explicitly for a more meaningful ``repr`` output.
+    :param bool auto_detect: Instead of setting the *init*, *repr*, *eq*,
+        *order*, and *hash* arguments explicitly, assume they are set to
+        ``True`` **unless any** of the involved methods for one of the
+        arguments is implemented in the *current* class (i.e. it is *not*
+        inherited from some base class).
+
+        So for example by implementing ``__eq__`` on a class yourself,
+        ``attrs`` will deduce ``eq=False`` and won't create *neither*
+        ``__eq__`` *nor* ``__ne__`` (but Python classes come with a sensible
+        ``__ne__`` by default, so it *should* be enough to only implement
+        ``__eq__`` in most cases).
+
+        .. warning::
+
+           If you prevent ``attrs`` from creating the ordering methods for you
+           (``order=False``, e.g. by implementing ``__le__``), it becomes
+           *your* responsibility to make sure its ordering is sound. The best
+           way is to use the `functools.total_ordering` decorator.
+
+
+        Passing ``True`` or ``False`` to *init*, *repr*, *eq*, *order*,
+        *cmp*, or *hash* overrides whatever *auto_detect* would determine.
+
+        *auto_detect* requires Python 3. Setting it ``True`` on Python 2 raises
+        a `PythonTooOldError`.
+
     :param bool repr: Create a ``__repr__`` method with a human readable
         representation of ``attrs`` attributes..
     :param bool str: Create a ``__str__`` method that is identical to
         ``__repr__``.  This is usually not necessary except for
         `Exception`\ s.
-    :param bool eq: If ``True`` or ``None`` (default), add ``__eq__`` and
-        ``__ne__`` methods that check two instances for equality.
+    :param Optional[bool] eq: If ``True`` or ``None`` (default), add ``__eq__``
+        and ``__ne__`` methods that check two instances for equality.
 
         They compare the instances as if they were tuples of their ``attrs``
-        attributes, but only iff the types of both classes are *identical*!
-    :type eq: `bool` or `None`
-    :param bool order: If ``True``, add ``__lt__``, ``__le__``, ``__gt__``,
-        and ``__ge__`` methods that behave like *eq* above and allow instances
-        to be ordered. If ``None`` (default) mirror value of *eq*.
-    :type order: `bool` or `None`
-    :param cmp: Setting to ``True`` is equivalent to setting ``eq=True,
-        order=True``. Deprecated in favor of *eq* and *order*, has precedence
-        over them for backward-compatibility though. Must not be mixed with
-        *eq* or *order*.
-    :type cmp: `bool` or `None`
-    :param hash: If ``None`` (default), the ``__hash__`` method is generated
-        according how *eq* and *frozen* are set.
+        attributes if and only if the types of both classes are *identical*!
+    :param Optional[bool] order: If ``True``, add ``__lt__``, ``__le__``,
+        ``__gt__``, and ``__ge__`` methods that behave like *eq* above and
+        allow instances to be ordered. If ``None`` (default) mirror value of
+        *eq*.
+    :param Optional[bool] cmp: Setting to ``True`` is equivalent to setting
+        ``eq=True, order=True``. Deprecated in favor of *eq* and *order*, has
+        precedence over them for backward-compatibility though. Must not be
+        mixed with *eq* or *order*.
+    :param Optional[bool] hash: If ``None`` (default), the ``__hash__`` method
+        is generated according how *eq* and *frozen* are set.
 
         1. If *both* are True, ``attrs`` will generate a ``__hash__`` for you.
         2. If *eq* is True and *frozen* is False, ``__hash__`` will be set to
@@ -845,18 +1120,19 @@ def attrs(
         `object.__hash__`, and the `GitHub issue that led to the default \
         behavior <https://github.com/python-attrs/attrs/issues/136>`_ for more
         details.
-    :type hash: ``bool`` or ``None``
     :param bool init: Create a ``__init__`` method that initializes the
         ``attrs`` attributes.  Leading underscores are stripped for the
         argument name.  If a ``__attrs_post_init__`` method exists on the
         class, it will be called after the class is fully initialized.
     :param bool slots: Create a `slotted class <slotted classes>` that's more
-        memory-efficient.
+        memory-efficient. Slotted classes are generally superior to the default
+        dict classes, but have some gotchas you should know about, so we
+        encourage you to read the `glossary entry <slotted classes>`.
     :param bool frozen: Make instances immutable after initialization.  If
         someone attempts to modify a frozen instance,
         `attr.exceptions.FrozenInstanceError` is raised.
 
-        Please note:
+        .. note::
 
             1. This is achieved by installing a custom ``__setattr__`` method
                on your class, so you can't implement your own.
@@ -872,10 +1148,12 @@ def attrs(
                circumvent that limitation by using
                ``object.__setattr__(self, "attribute_name", value)``.
 
+            5. Subclasses of a frozen class are frozen too.
+
     :param bool weakref_slot: Make instances weak-referenceable.  This has no
         effect unless ``slots`` is also enabled.
-    :param bool auto_attribs: If True, collect `PEP 526`_-annotated attributes
-        (Python 3.6 and later only) from the class body.
+    :param bool auto_attribs: If ``True``, collect `PEP 526`_-annotated
+        attributes (Python 3.6 and later only) from the class body.
 
         In this case, you **must** annotate every field.  If ``attrs``
         encounters a field that is set to an `attr.ib` but lacks a type
@@ -915,6 +1193,46 @@ def attrs(
           default value are additionally available as a tuple in the ``args``
           attribute,
         - the value of *str* is ignored leaving ``__str__`` to base classes.
+    :param bool collect_by_mro: Setting this to `True` fixes the way ``attrs``
+       collects attributes from base classes.  The default behavior is
+       incorrect in certain cases of multiple inheritance.  It should be on by
+       default but is kept off for backward-compatability.
+
+       See issue `#428 <https://github.com/python-attrs/attrs/issues/428>`_ for
+       more details.
+
+    :param Optional[bool] getstate_setstate:
+       .. note::
+          This is usually only interesting for slotted classes and you should
+          probably just set *auto_detect* to `True`.
+
+       If `True`, ``__getstate__`` and
+       ``__setstate__`` are generated and attached to the class. This is
+       necessary for slotted classes to be pickleable. If left `None`, it's
+       `True` by default for slotted classes and ``False`` for dict classes.
+
+       If *auto_detect* is `True`, and *getstate_setstate* is left `None`,
+       and **either** ``__getstate__`` or ``__setstate__`` is detected directly
+       on the class (i.e. not inherited), it is set to `False` (this is usually
+       what you want).
+
+    :param on_setattr: A callable that is run whenever the user attempts to set
+        an attribute (either by assignment like ``i.x = 42`` or by using
+        `setattr` like ``setattr(i, "x", 42)``). It receives the same arguments
+        as validators: the instance, the attribute that is being modified, and
+        the new value.
+
+        If no exception is raised, the attribute is set to the return value of
+        the callable.
+
+        If a list of callables is passed, they're automatically wrapped in an
+        `attr.setters.pipe`.
+
+    :param Optional[callable] field_transformer:
+        A function that is called with the original class object and all
+        fields right before ``attrs`` finalizes the class.  You can use
+        this, e.g., to automatically add converters or validators to
+        fields based on their types.  See `transform-fields` for more details.
 
     .. versionadded:: 16.0.0 *slots*
     .. versionadded:: 16.1.0 *frozen*
@@ -940,37 +1258,86 @@ def attrs(
     .. versionadded:: 19.1.0 *auto_exc*
     .. deprecated:: 19.2.0 *cmp* Removal on or after 2021-06-01.
     .. versionadded:: 19.2.0 *eq* and *order*
+    .. versionadded:: 20.1.0 *auto_detect*
+    .. versionadded:: 20.1.0 *collect_by_mro*
+    .. versionadded:: 20.1.0 *getstate_setstate*
+    .. versionadded:: 20.1.0 *on_setattr*
+    .. versionadded:: 20.3.0 *field_transformer*
     """
-    eq, order = _determine_eq_order(cmp, eq, order)
+    if auto_detect and PY2:
+        raise PythonTooOldError(
+            "auto_detect only works on Python 3 and later."
+        )
+
+    eq_, order_ = _determine_eq_order(cmp, eq, order, None)
+    hash_ = hash  # work around the lack of nonlocal
+
+    if isinstance(on_setattr, (list, tuple)):
+        on_setattr = setters.pipe(*on_setattr)
 
     def wrap(cls):
 
         if getattr(cls, "__class__", None) is None:
             raise TypeError("attrs only works with new-style classes.")
 
+        is_frozen = frozen or _has_frozen_base_class(cls)
         is_exc = auto_exc is True and issubclass(cls, BaseException)
+        has_own_setattr = auto_detect and _has_own_attribute(
+            cls, "__setattr__"
+        )
+
+        if has_own_setattr and is_frozen:
+            raise ValueError("Can't freeze a class with a custom __setattr__.")
 
         builder = _ClassBuilder(
             cls,
             these,
             slots,
-            frozen,
+            is_frozen,
             weakref_slot,
+            _determine_whether_to_implement(
+                cls,
+                getstate_setstate,
+                auto_detect,
+                ("__getstate__", "__setstate__"),
+                default=slots,
+            ),
             auto_attribs,
             kw_only,
             cache_hash,
             is_exc,
+            collect_by_mro,
+            on_setattr,
+            has_own_setattr,
+            field_transformer,
         )
-
-        if repr is True:
+        if _determine_whether_to_implement(
+            cls, repr, auto_detect, ("__repr__",)
+        ):
             builder.add_repr(repr_ns)
         if str is True:
             builder.add_str()
-        if eq is True and not is_exc:
+
+        eq = _determine_whether_to_implement(
+            cls, eq_, auto_detect, ("__eq__", "__ne__")
+        )
+        if not is_exc and eq is True:
             builder.add_eq()
-        if order is True and not is_exc:
+        if not is_exc and _determine_whether_to_implement(
+            cls, order_, auto_detect, ("__lt__", "__le__", "__gt__", "__ge__")
+        ):
             builder.add_order()
 
+        builder.add_setattr()
+
+        if (
+            hash_ is None
+            and auto_detect is True
+            and _has_own_attribute(cls, "__hash__")
+        ):
+            hash = False
+        else:
+            hash = hash_
         if hash is not True and hash is not False and hash is not None:
             # Can't use `hash in` because 1 == True for example.
             raise TypeError(
@@ -985,7 +1352,9 @@ def wrap(cls):
                     " hashing must be either explicitly or implicitly "
                     "enabled."
                 )
-        elif hash is True or (hash is None and eq is True and frozen is True):
+        elif hash is True or (
+            hash is None and eq is True and is_frozen is True
+        ):
             # Build a __hash__ if told so, or if it's safe.
             builder.add_hash()
         else:
@@ -998,7 +1367,9 @@ def wrap(cls):
                 )
             builder.make_unhashable()
 
-        if init is True:
+        if _determine_whether_to_implement(
+            cls, init, auto_detect, ("__init__",)
+        ):
             builder.add_init()
         else:
             if cache_hash:
@@ -1095,7 +1466,23 @@ def _make_hash(cls, attrs, frozen, cache_hash):
     unique_filename = _generate_unique_filename(cls, "hash")
     type_hash = hash(unique_filename)
 
-    method_lines = ["def __hash__(self):"]
+    hash_def = "def __hash__(self"
+    hash_func = "hash(("
+    closing_braces = "))"
+    if not cache_hash:
+        hash_def += "):"
+    else:
+        if not PY2:
+            hash_def += ", *"
+
+        hash_def += (
+            ", _cache_wrapper="
+            + "__import__('attr._make')._make._CacheHashWrapper):"
+        )
+        hash_func = "_cache_wrapper(" + hash_func
+        closing_braces += ")"
+
+    method_lines = [hash_def]
 
     def append_hash_computation_lines(prefix, indent):
         """
@@ -1103,14 +1490,18 @@ def append_hash_computation_lines(prefix, indent):
         Below this will either be returned directly or used to compute
         a value which is then cached, depending on the value of cache_hash
         """
+
         method_lines.extend(
-            [indent + prefix + "hash((", indent + "        %d," % (type_hash,)]
+            [
+                indent + prefix + hash_func,
+                indent + "        %d," % (type_hash,),
+            ]
         )
 
         for a in attrs:
             method_lines.append(indent + "        self.%s," % a.name)
 
-        method_lines.append(indent + "    ))")
+        method_lines.append(indent + "    " + closing_braces)
 
     if cache_hash:
         method_lines.append(tab + "if self.%s is None:" % _hash_cache_field)
@@ -1153,19 +1544,29 @@ def _add_hash(cls, attrs):
     return cls
 
 
-def __ne__(self, other):
+def _make_ne():
     """
-    Check equality and either forward a NotImplemented or return the result
-    negated.
+    Create __ne__ method.
     """
-    result = self.__eq__(other)
-    if result is NotImplemented:
-        return NotImplemented
 
-    return not result
+    def __ne__(self, other):
+        """
+        Check equality and either forward a NotImplemented or
+        return the result negated.
+        """
+        result = self.__eq__(other)
+        if result is NotImplemented:
+            return NotImplemented
+
+        return not result
+
+    return __ne__
 
 
 def _make_eq(cls, attrs):
+    """
+    Create __eq__ method for *cls* with *attrs*.
+    """
     attrs = [a for a in attrs if a.eq]
 
     unique_filename = _generate_unique_filename(cls, "eq")
@@ -1201,10 +1602,13 @@ def _make_eq(cls, attrs):
         script.splitlines(True),
         unique_filename,
     )
-    return locs["__eq__"], __ne__
+    return locs["__eq__"]
 
 
 def _make_order(cls, attrs):
+    """
+    Create ordering methods for *cls* with *attrs*.
+    """
     attrs = [a for a in attrs if a.order]
 
     def attrs_to_tuple(obj):
@@ -1259,7 +1663,8 @@ def _add_eq(cls, attrs=None):
     if attrs is None:
         attrs = cls.__attrs_attrs__
 
-    cls.__eq__, cls.__ne__ = _make_eq(cls, attrs)
+    cls.__eq__ = _make_eq(cls, attrs)
+    cls.__ne__ = _make_ne()
 
     return cls
 
@@ -1337,43 +1742,6 @@ def _add_repr(cls, ns=None, attrs=None):
     return cls
 
 
-def _make_init(
-    cls, attrs, post_init, frozen, slots, cache_hash, base_attr_map, is_exc
-):
-    attrs = [a for a in attrs if a.init or a.default is not NOTHING]
-
-    unique_filename = _generate_unique_filename(cls, "init")
-
-    script, globs, annotations = _attrs_to_init_script(
-        attrs, frozen, slots, post_init, cache_hash, base_attr_map, is_exc
-    )
-    locs = {}
-    bytecode = compile(script, unique_filename, "exec")
-    attr_dict = dict((a.name, a) for a in attrs)
-    globs.update({"NOTHING": NOTHING, "attr_dict": attr_dict})
-
-    if frozen is True:
-        # Save the lookup overhead in __init__ if we need to circumvent
-        # immutability.
-        globs["_cached_setattr"] = _obj_setattr
-
-    eval(bytecode, globs, locs)
-
-    # In order of debuggers like PDB being able to step through the code,
-    # we add a fake linecache entry.
-    linecache.cache[unique_filename] = (
-        len(script),
-        None,
-        script.splitlines(True),
-        unique_filename,
-    )
-
-    __init__ = locs["__init__"]
-    __init__.__annotations__ = annotations
-
-    return __init__
-
-
 def fields(cls):
     """
     Return the tuple of ``attrs`` attributes for a class.
@@ -1458,8 +1826,191 @@ def _is_slot_attr(a_name, base_attr_map):
     return a_name in base_attr_map and _is_slot_cls(base_attr_map[a_name])
 
 
+def _make_init(
+    cls,
+    attrs,
+    post_init,
+    frozen,
+    slots,
+    cache_hash,
+    base_attr_map,
+    is_exc,
+    has_global_on_setattr,
+):
+    if frozen and has_global_on_setattr:
+        raise ValueError("Frozen classes can't use on_setattr.")
+
+    needs_cached_setattr = cache_hash or frozen
+    filtered_attrs = []
+    attr_dict = {}
+    for a in attrs:
+        if not a.init and a.default is NOTHING:
+            continue
+
+        filtered_attrs.append(a)
+        attr_dict[a.name] = a
+
+        if a.on_setattr is not None:
+            if frozen is True:
+                raise ValueError("Frozen classes can't use on_setattr.")
+
+            needs_cached_setattr = True
+        elif (
+            has_global_on_setattr and a.on_setattr is not setters.NO_OP
+        ) or _is_slot_attr(a.name, base_attr_map):
+            needs_cached_setattr = True
+
+    unique_filename = _generate_unique_filename(cls, "init")
+
+    script, globs, annotations = _attrs_to_init_script(
+        filtered_attrs,
+        frozen,
+        slots,
+        post_init,
+        cache_hash,
+        base_attr_map,
+        is_exc,
+        needs_cached_setattr,
+        has_global_on_setattr,
+    )
+    locs = {}
+    bytecode = compile(script, unique_filename, "exec")
+    globs.update({"NOTHING": NOTHING, "attr_dict": attr_dict})
+
+    if needs_cached_setattr:
+        # Save the lookup overhead in __init__ if we need to circumvent
+        # setattr hooks.
+        globs["_cached_setattr"] = _obj_setattr
+
+    eval(bytecode, globs, locs)
+
+    # In order of debuggers like PDB being able to step through the code,
+    # we add a fake linecache entry.
+    linecache.cache[unique_filename] = (
+        len(script),
+        None,
+        script.splitlines(True),
+        unique_filename,
+    )
+
+    __init__ = locs["__init__"]
+    __init__.__annotations__ = annotations
+
+    return __init__
+
+
+def _setattr(attr_name, value_var, has_on_setattr):
+    """
+    Use the cached object.setattr to set *attr_name* to *value_var*.
+    """
+    return "_setattr('%s', %s)" % (attr_name, value_var)
+
+
+def _setattr_with_converter(attr_name, value_var, has_on_setattr):
+    """
+    Use the cached object.setattr to set *attr_name* to *value_var*, but run
+    its converter first.
+    """
+    return "_setattr('%s', %s(%s))" % (
+        attr_name,
+        _init_converter_pat % (attr_name,),
+        value_var,
+    )
+
+
+def _assign(attr_name, value, has_on_setattr):
+    """
+    Unless *attr_name* has an on_setattr hook, use normal assignment. Otherwise
+    relegate to _setattr.
+    """
+    if has_on_setattr:
+        return _setattr(attr_name, value, True)
+
+    return "self.%s = %s" % (attr_name, value)
+
+
+def _assign_with_converter(attr_name, value_var, has_on_setattr):
+    """
+    Unless *attr_name* has an on_setattr hook, use normal assignment after
+    conversion. Otherwise relegate to _setattr_with_converter.
+    """
+    if has_on_setattr:
+        return _setattr_with_converter(attr_name, value_var, True)
+
+    return "self.%s = %s(%s)" % (
+        attr_name,
+        _init_converter_pat % (attr_name,),
+        value_var,
+    )
+
+
+if PY2:
+
+    def _unpack_kw_only_py2(attr_name, default=None):
+        """
+        Unpack *attr_name* from _kw_only dict.
+        """
+        if default is not None:
+            arg_default = ", %s" % default
+        else:
+            arg_default = ""
+        return "%s = _kw_only.pop('%s'%s)" % (
+            attr_name,
+            attr_name,
+            arg_default,
+        )
+
+    def _unpack_kw_only_lines_py2(kw_only_args):
+        """
+        Unpack all *kw_only_args* from _kw_only dict and handle errors.
+
+        Given a list of strings "{attr_name}" and "{attr_name}={default}"
+        generates list of lines of code that pop attrs from _kw_only dict and
+        raise TypeError similar to builtin if required attr is missing or
+        extra key is passed.
+
+        >>> print("\n".join(_unpack_kw_only_lines_py2(["a", "b=42"])))
+        try:
+            a = _kw_only.pop('a')
+            b = _kw_only.pop('b', 42)
+        except KeyError as _key_error:
+            raise TypeError(
+                ...
+        if _kw_only:
+            raise TypeError(
+                ...
+        """
+        lines = ["try:"]
+        lines.extend(
+            "    " + _unpack_kw_only_py2(*arg.split("="))
+            for arg in kw_only_args
+        )
+        lines += """\
+except KeyError as _key_error:
+    raise TypeError(
+        '__init__() missing required keyword-only argument: %s' % _key_error
+    )
+if _kw_only:
+    raise TypeError(
+        '__init__() got an unexpected keyword argument %r'
+        % next(iter(_kw_only))
+    )
+""".split(
+            "\n"
+        )
+        return lines
+
+
 def _attrs_to_init_script(
-    attrs, frozen, slots, post_init, cache_hash, base_attr_map, is_exc
+    attrs,
+    frozen,
+    slots,
+    post_init,
+    cache_hash,
+    base_attr_map,
+    is_exc,
+    needs_cached_setattr,
+    has_global_on_setattr,
 ):
     """
     Return a script of an initializer for *attrs* and a dict of globals.
@@ -1470,85 +2021,49 @@ def _attrs_to_init_script(
     a cached ``object.__setattr__``.
     """
     lines = []
-    any_slot_ancestors = any(
-        _is_slot_attr(a.name, base_attr_map) for a in attrs
-    )
+    if needs_cached_setattr:
+        lines.append(
+            # Circumvent the __setattr__ descriptor to save one lookup per
+            # assignment.
+            # Note _setattr will be used again below if cache_hash is True
+            "_setattr = _cached_setattr.__get__(self, self.__class__)"
+        )
+
     if frozen is True:
         if slots is True:
-            lines.append(
-                # Circumvent the __setattr__ descriptor to save one lookup per
-                # assignment.
-                # Note _setattr will be used again below if cache_hash is True
-                "_setattr = _cached_setattr.__get__(self, self.__class__)"
-            )
-
-            def fmt_setter(attr_name, value_var):
-                return "_setattr('%(attr_name)s', %(value_var)s)" % {
-                    "attr_name": attr_name,
-                    "value_var": value_var,
-                }
-
-            def fmt_setter_with_converter(attr_name, value_var):
-                conv_name = _init_converter_pat.format(attr_name)
-                return "_setattr('%(attr_name)s', %(conv)s(%(value_var)s))" % {
-                    "attr_name": attr_name,
-                    "value_var": value_var,
-                    "conv": conv_name,
-                }
-
+            fmt_setter = _setattr
+            fmt_setter_with_converter = _setattr_with_converter
         else:
             # Dict frozen classes assign directly to __dict__.
             # But only if the attribute doesn't come from an ancestor slot
             # class.
             # Note _inst_dict will be used again below if cache_hash is True
             lines.append("_inst_dict = self.__dict__")
-            if any_slot_ancestors:
-                lines.append(
-                    # Circumvent the __setattr__ descriptor to save one lookup
-                    # per assignment.
-                    "_setattr = _cached_setattr.__get__(self, self.__class__)"
-                )
 
-            def fmt_setter(attr_name, value_var):
-                if _is_slot_attr(attr_name, base_attr_map):
-                    res = "_setattr('%(attr_name)s', %(value_var)s)" % {
-                        "attr_name": attr_name,
-                        "value_var": value_var,
-                    }
-                else:
-                    res = "_inst_dict['%(attr_name)s'] = %(value_var)s" % {
-                        "attr_name": attr_name,
-                        "value_var": value_var,
-                    }
-                return res
-
-            def fmt_setter_with_converter(attr_name, value_var):
-                conv_name = _init_converter_pat.format(attr_name)
+            def fmt_setter(attr_name, value_var, has_on_setattr):
                 if _is_slot_attr(attr_name, base_attr_map):
-                    tmpl = "_setattr('%(attr_name)s', %(c)s(%(value_var)s))"
-                else:
-                    tmpl = "_inst_dict['%(attr_name)s'] = %(c)s(%(value_var)s)"
-                return tmpl % {
-                    "attr_name": attr_name,
-                    "value_var": value_var,
-                    "c": conv_name,
-                }
+                    return _setattr(attr_name, value_var, has_on_setattr)
+
+                return "_inst_dict['%s'] = %s" % (attr_name, value_var)
+
+            def fmt_setter_with_converter(
+                attr_name, value_var, has_on_setattr
+            ):
+                if has_on_setattr or _is_slot_attr(attr_name, base_attr_map):
+                    return _setattr_with_converter(
+                        attr_name, value_var, has_on_setattr
+                    )
+
+                return "_inst_dict['%s'] = %s(%s)" % (
+                    attr_name,
+                    _init_converter_pat % (attr_name,),
+                    value_var,
+                )
 
     else:
         # Not frozen.
-        def fmt_setter(attr_name, value):
-            return "self.%(attr_name)s = %(value)s" % {
-                "attr_name": attr_name,
-                "value": value,
-            }
-
-        def fmt_setter_with_converter(attr_name, value_var):
-            conv_name = _init_converter_pat.format(attr_name)
-            return "self.%(attr_name)s = %(conv)s(%(value_var)s)" % {
-                "attr_name": attr_name,
-                "value_var": value_var,
-                "conv": conv_name,
-            }
+        fmt_setter = _assign
+        fmt_setter_with_converter = _assign_with_converter
 
     args = []
     kw_only_args = []
@@ -1562,13 +2077,19 @@ def fmt_setter_with_converter(attr_name, value_var):
     for a in attrs:
         if a.validator:
             attrs_to_validate.append(a)
+
         attr_name = a.name
+        has_on_setattr = a.on_setattr is not None or (
+            a.on_setattr is not setters.NO_OP and has_global_on_setattr
+        )
         arg_name = a.name.lstrip("_")
+
         has_factory = isinstance(a.default, Factory)
         if has_factory and a.default.takes_self:
             maybe_self = "self"
         else:
             maybe_self = ""
+
         if a.init is False:
             if has_factory:
                 init_factory_name = _init_factory_pat.format(a.name)
@@ -1576,16 +2097,18 @@ def fmt_setter_with_converter(attr_name, value_var):
                     lines.append(
                         fmt_setter_with_converter(
                             attr_name,
-                            init_factory_name + "({0})".format(maybe_self),
+                            init_factory_name + "(%s)" % (maybe_self,),
+                            has_on_setattr,
                         )
                     )
-                    conv_name = _init_converter_pat.format(a.name)
+                    conv_name = _init_converter_pat % (a.name,)
                     names_for_globals[conv_name] = a.converter
                 else:
                     lines.append(
                         fmt_setter(
                             attr_name,
-                            init_factory_name + "({0})".format(maybe_self),
+                            init_factory_name + "(%s)" % (maybe_self,),
+                            has_on_setattr,
                         )
                     )
                 names_for_globals[init_factory_name] = a.default.factory
@@ -1594,70 +2117,78 @@ def fmt_setter_with_converter(attr_name, value_var):
                     lines.append(
                         fmt_setter_with_converter(
                             attr_name,
-                            "attr_dict['{attr_name}'].default".format(
-                                attr_name=attr_name
-                            ),
+                            "attr_dict['%s'].default" % (attr_name,),
+                            has_on_setattr,
                         )
                     )
-                    conv_name = _init_converter_pat.format(a.name)
+                    conv_name = _init_converter_pat % (a.name,)
                     names_for_globals[conv_name] = a.converter
                 else:
                     lines.append(
                         fmt_setter(
                             attr_name,
-                            "attr_dict['{attr_name}'].default".format(
-                                attr_name=attr_name
-                            ),
+                            "attr_dict['%s'].default" % (attr_name,),
+                            has_on_setattr,
                         )
                     )
         elif a.default is not NOTHING and not has_factory:
-            arg = "{arg_name}=attr_dict['{attr_name}'].default".format(
-                arg_name=arg_name, attr_name=attr_name
-            )
+            arg = "%s=attr_dict['%s'].default" % (arg_name, attr_name)
             if a.kw_only:
                 kw_only_args.append(arg)
             else:
                 args.append(arg)
+
             if a.converter is not None:
-                lines.append(fmt_setter_with_converter(attr_name, arg_name))
+                lines.append(
+                    fmt_setter_with_converter(
+                        attr_name, arg_name, has_on_setattr
+                    )
+                )
                 names_for_globals[
-                    _init_converter_pat.format(a.name)
+                    _init_converter_pat % (a.name,)
                 ] = a.converter
             else:
-                lines.append(fmt_setter(attr_name, arg_name))
+                lines.append(fmt_setter(attr_name, arg_name, has_on_setattr))
+
         elif has_factory:
-            arg = "{arg_name}=NOTHING".format(arg_name=arg_name)
+            arg = "%s=NOTHING" % (arg_name,)
             if a.kw_only:
                 kw_only_args.append(arg)
             else:
                 args.append(arg)
-            lines.append(
-                "if {arg_name} is not NOTHING:".format(arg_name=arg_name)
-            )
+            lines.append("if %s is not NOTHING:" % (arg_name,))
+
             init_factory_name = _init_factory_pat.format(a.name)
             if a.converter is not None:
                 lines.append(
-                    "    " + fmt_setter_with_converter(attr_name, arg_name)
+                    "    "
+                    + fmt_setter_with_converter(
+                        attr_name, arg_name, has_on_setattr
+                    )
                 )
                 lines.append("else:")
                 lines.append(
                     "    "
                     + fmt_setter_with_converter(
                         attr_name,
-                        init_factory_name + "({0})".format(maybe_self),
+                        init_factory_name + "(" + maybe_self + ")",
+                        has_on_setattr,
                     )
                 )
                 names_for_globals[
-                    _init_converter_pat.format(a.name)
+                    _init_converter_pat % (a.name,)
                 ] = a.converter
             else:
-                lines.append("    " + fmt_setter(attr_name, arg_name))
+                lines.append(
+                    "    " + fmt_setter(attr_name, arg_name, has_on_setattr)
+                )
                 lines.append("else:")
                 lines.append(
                     "    "
                     + fmt_setter(
                         attr_name,
-                        init_factory_name + "({0})".format(maybe_self),
+                        init_factory_name + "(" + maybe_self + ")",
+                        has_on_setattr,
                     )
                 )
             names_for_globals[init_factory_name] = a.default.factory
@@ -1666,13 +2197,18 @@ def fmt_setter_with_converter(attr_name, value_var):
                 kw_only_args.append(arg_name)
             else:
                 args.append(arg_name)
+
             if a.converter is not None:
-                lines.append(fmt_setter_with_converter(attr_name, arg_name))
+                lines.append(
+                    fmt_setter_with_converter(
+                        attr_name, arg_name, has_on_setattr
+                    )
+                )
                 names_for_globals[
-                    _init_converter_pat.format(a.name)
+                    _init_converter_pat % (a.name,)
                 ] = a.converter
             else:
-                lines.append(fmt_setter(attr_name, arg_name))
+                lines.append(fmt_setter(attr_name, arg_name, has_on_setattr))
 
         if a.init is True and a.converter is None and a.type is not None:
             annotations[arg_name] = a.type
@@ -1681,13 +2217,14 @@ def fmt_setter_with_converter(attr_name, value_var):
         names_for_globals["_config"] = _config
         lines.append("if _config._run_validators is True:")
         for a in attrs_to_validate:
-            val_name = "__attr_validator_{}".format(a.name)
-            attr_name = "__attr_{}".format(a.name)
+            val_name = "__attr_validator_" + a.name
+            attr_name = "__attr_" + a.name
             lines.append(
-                "    {}(self, {}, self.{})".format(val_name, attr_name, a.name)
+                "    %s(self, %s, self.%s)" % (val_name, attr_name, a.name)
             )
             names_for_globals[val_name] = a.validator
             names_for_globals[attr_name] = a
+
     if post_init:
         lines.append("self.__attrs_post_init__()")
 
@@ -1718,14 +2255,14 @@ def fmt_setter_with_converter(attr_name, value_var):
     args = ", ".join(args)
     if kw_only_args:
         if PY2:
-            raise PythonTooOldError(
-                "Keyword-only arguments only work on Python 3 and later."
-            )
+            lines = _unpack_kw_only_lines_py2(kw_only_args) + lines
 
-        args += "{leading_comma}*, {kw_only_args}".format(
-            leading_comma=", " if args else "",
-            kw_only_args=", ".join(kw_only_args),
-        )
+            args += "%s**_kw_only" % (", " if args else "",)  # leading comma
+        else:
+            args += "%s*, %s" % (
+                ", " if args else "",  # leading comma
+                ", ".join(kw_only_args),  # kw_only args
+            )
     return (
         """\
 def __init__(self, {args}):
@@ -1742,12 +2279,26 @@ class Attribute(object):
     """
     *Read-only* representation of an attribute.
 
+    Instances of this class are frequently used for introspection purposes
+    like:
+
+    - `fields` returns a tuple of them.
+    - Validators get them passed as the first argument.
+    - The *field transformer* hook receives a list of them.
+
     :attribute name: The name of the attribute.
+    :attribute inherited: Whether or not that attribute has been inherited from
+        a base class.
 
     Plus *all* arguments of `attr.ib` (except for ``factory``
     which is only syntactic sugar for ``default=Factory(...)``.
 
-    For the version history of the fields, see `attr.ib`.
+    .. versionadded:: 20.1.0 *inherited*
+    .. versionadded:: 20.1.0 *on_setattr*
+    .. versionchanged:: 20.2.0 *inherited* is not taken into account for
+        equality checks and hashing anymore.
+
+    For the full version history of the fields, see `attr.ib`.
     """
 
     __slots__ = (
@@ -1763,6 +2314,8 @@ class Attribute(object):
         "type",
         "converter",
         "kw_only",
+        "inherited",
+        "on_setattr",
     )
 
     def __init__(
@@ -1774,14 +2327,16 @@ def __init__(
         cmp,  # XXX: unused, remove along with other cmp code.
         hash,
         init,
+        inherited,
         metadata=None,
         type=None,
         converter=None,
         kw_only=False,
         eq=None,
         order=None,
+        on_setattr=None,
     ):
-        eq, order = _determine_eq_order(cmp, eq, order)
+        eq, order = _determine_eq_order(cmp, eq, order, True)
 
         # Cache this descriptor here to speed things up later.
         bound_setattr = _obj_setattr.__get__(self, Attribute)
@@ -1807,6 +2362,8 @@ def __init__(
         )
         bound_setattr("type", type)
         bound_setattr("kw_only", kw_only)
+        bound_setattr("inherited", inherited)
+        bound_setattr("on_setattr", on_setattr)
 
     def __setattr__(self, name, value):
         raise FrozenInstanceError()
@@ -1829,6 +2386,7 @@ def from_counting_attr(cls, name, ca, type=None):
                 "validator",
                 "default",
                 "type",
+                "inherited",
             )  # exclude methods and deprecated alias
         }
         return cls(
@@ -1837,6 +2395,7 @@ def from_counting_attr(cls, name, ca, type=None):
             default=ca._default,
             type=type,
             cmp=None,
+            inherited=False,
             **inst_dict
         )
 
@@ -1849,10 +2408,17 @@ def cmp(self):
 
         return self.eq and self.order
 
-    # Don't use attr.assoc since fields(Attribute) doesn't work
-    def _assoc(self, **changes):
+    # Don't use attr.evolve since fields(Attribute) doesn't work
+    def evolve(self, **changes):
         """
         Copy *self* and apply *changes*.
+
+        This works similarly to `attr.evolve` but that function does not work
+        with ``Attribute``.
+
+        It is mainly meant to be used for `transform-fields`.
+
+        .. versionadded:: 20.3.0
         """
         new = copy.copy(self)
 
@@ -1901,13 +2467,17 @@ def _setattrs(self, name_values_pairs):
         order=False,
         hash=(name != "metadata"),
         init=True,
+        inherited=False,
     )
     for name in Attribute.__slots__
 ]
 
 Attribute = _add_hash(
-    _add_eq(_add_repr(Attribute, attrs=_a), attrs=_a),
-    attrs=[a for a in _a if a.hash],
+    _add_eq(
+        _add_repr(Attribute, attrs=_a),
+        attrs=[a for a in _a if a.name != "inherited"],
+    ),
+    attrs=[a for a in _a if a.hash and a.name != "inherited"],
 )
 
 
@@ -1933,6 +2503,7 @@ class _CountingAttr(object):
         "converter",
         "type",
         "kw_only",
+        "on_setattr",
     )
     __attrs_attrs__ = tuple(
         Attribute(
@@ -1946,6 +2517,8 @@ class _CountingAttr(object):
             kw_only=False,
             eq=True,
             order=False,
+            inherited=False,
+            on_setattr=None,
         )
         for name in (
             "counter",
@@ -1955,6 +2528,7 @@ class _CountingAttr(object):
             "order",
             "hash",
             "init",
+            "on_setattr",
         )
     ) + (
         Attribute(
@@ -1968,6 +2542,8 @@ class _CountingAttr(object):
             kw_only=False,
             eq=True,
             order=False,
+            inherited=False,
+            on_setattr=None,
         ),
     )
     cls_counter = 0
@@ -1986,24 +2562,22 @@ def __init__(
         kw_only,
         eq,
         order,
+        on_setattr,
     ):
         _CountingAttr.cls_counter += 1
         self.counter = _CountingAttr.cls_counter
         self._default = default
-        # If validator is a list/tuple, wrap it using helper validator.
-        if validator and isinstance(validator, (list, tuple)):
-            self._validator = and_(*validator)
-        else:
-            self._validator = validator
+        self._validator = validator
+        self.converter = converter
         self.repr = repr
         self.eq = eq
         self.order = order
         self.hash = hash
         self.init = init
-        self.converter = converter
         self.metadata = metadata
         self.type = type
         self.kw_only = kw_only
+        self.on_setattr = on_setattr
 
     def validator(self, meth):
         """
@@ -2072,8 +2646,7 @@ def make_class(name, attrs, bases=(object,), **attributes_arguments):
     """
     A quick way to create a new class called *name* with *attrs*.
 
-    :param name: The name for the new class.
-    :type name: str
+    :param str name: The name for the new class.
 
     :param attrs: A list of names or a dictionary of mappings of names to
         attributes.
@@ -2120,17 +2693,21 @@ def make_class(name, attrs, bases=(object,), **attributes_arguments):
 
     # We do it here for proper warnings with meaningful stacklevel.
     cmp = attributes_arguments.pop("cmp", None)
-    attributes_arguments["eq"], attributes_arguments[
-        "order"
-    ] = _determine_eq_order(
-        cmp, attributes_arguments.get("eq"), attributes_arguments.get("order")
+    (
+        attributes_arguments["eq"],
+        attributes_arguments["order"],
+    ) = _determine_eq_order(
+        cmp,
+        attributes_arguments.get("eq"),
+        attributes_arguments.get("order"),
+        True,
     )
 
     return _attrs(these=cls_dict, **attributes_arguments)(type_)
 
 
 # These are required by within this module so we define them here and merely
-# import into .validators.
+# import into .validators / .converters.
 
 
 @attrs(slots=True, hash=True)
@@ -2152,8 +2729,7 @@ def and_(*validators):
 
     When called on a value, it runs all wrapped validators.
 
-    :param validators: Arbitrary number of validators.
-    :type validators: callables
+    :param callables validators: Arbitrary number of validators.
 
     .. versionadded:: 17.1.0
     """
@@ -2166,3 +2742,24 @@ def and_(*validators):
         )
 
     return _AndValidator(tuple(vals))
+
+
+def pipe(*converters):
+    """
+    A converter that composes multiple converters into one.
+
+    When called on a value, it runs all wrapped converters, returning the
+    *last* value.
+
+    :param callables converters: Arbitrary number of converters.
+
+    .. versionadded:: 20.1.0
+    """
+
+    def pipe_converter(val):
+        for converter in converters:
+            val = converter(val)
+
+        return val
+
+    return pipe_converter
diff --git a/poetry/core/_vendor/attr/_next_gen.py b/poetry/core/_vendor/attr/_next_gen.py
new file mode 100644
index 000000000..2b5565c56
--- /dev/null
+++ b/poetry/core/_vendor/attr/_next_gen.py
@@ -0,0 +1,160 @@
+"""
+This is a Python 3.6 and later-only, keyword-only, and **provisional** API that
+calls `attr.s` with different default values.
+
+Provisional APIs that shall become "import attrs" one glorious day.
+"""
+
+from functools import partial
+
+from attr.exceptions import UnannotatedAttributeError
+
+from . import setters
+from ._make import NOTHING, _frozen_setattrs, attrib, attrs
+
+
+def define(
+    maybe_cls=None,
+    *,
+    these=None,
+    repr=None,
+    hash=None,
+    init=None,
+    slots=True,
+    frozen=False,
+    weakref_slot=True,
+    str=False,
+    auto_attribs=None,
+    kw_only=False,
+    cache_hash=False,
+    auto_exc=True,
+    eq=None,
+    order=False,
+    auto_detect=True,
+    getstate_setstate=None,
+    on_setattr=None,
+    field_transformer=None,
+):
+    r"""
+    The only behavioral differences are the handling of the *auto_attribs*
+    option:
+
+    :param Optional[bool] auto_attribs: If set to `True` or `False`, it behaves
+       exactly like `attr.s`. If left `None`, `attr.s` will try to guess:
+
+       1. If all attributes are annotated and no `attr.ib` is found, it assumes
+          *auto_attribs=True*.
+       2. Otherwise it assumes *auto_attribs=False* and tries to collect
+          `attr.ib`\ s.
+
+    and that mutable classes (``frozen=False``) validate on ``__setattr__``.
+
+    .. versionadded:: 20.1.0
+    """
+
+    def do_it(cls, auto_attribs):
+        return attrs(
+            maybe_cls=cls,
+            these=these,
+            repr=repr,
+            hash=hash,
+            init=init,
+            slots=slots,
+            frozen=frozen,
+            weakref_slot=weakref_slot,
+            str=str,
+            auto_attribs=auto_attribs,
+            kw_only=kw_only,
+            cache_hash=cache_hash,
+            auto_exc=auto_exc,
+            eq=eq,
+            order=order,
+            auto_detect=auto_detect,
+            collect_by_mro=True,
+            getstate_setstate=getstate_setstate,
+            on_setattr=on_setattr,
+            field_transformer=field_transformer,
+        )
+
+    def wrap(cls):
+        """
+        Making this a wrapper ensures this code runs during class creation.
+
+        We also ensure that frozen-ness of classes is inherited.
+        """
+        nonlocal frozen, on_setattr
+
+        had_on_setattr = on_setattr not in (None, setters.NO_OP)
+
+        # By default, mutable classes validate on setattr.
+        if frozen is False and on_setattr is None:
+            on_setattr = setters.validate
+
+        # However, if we subclass a frozen class, we inherit the immutability
+        # and disable on_setattr.
+        for base_cls in cls.__bases__:
+            if base_cls.__setattr__ is _frozen_setattrs:
+                if had_on_setattr:
+                    raise ValueError(
+                        "Frozen classes can't use on_setattr "
+                        "(frozen-ness was inherited)."
+                    )
+
+                on_setattr = setters.NO_OP
+                break
+
+        if auto_attribs is not None:
+            return do_it(cls, auto_attribs)
+
+        try:
+            return do_it(cls, True)
+        except UnannotatedAttributeError:
+            return do_it(cls, False)
+
+    # maybe_cls's type depends on the usage of the decorator.  It's a class
+    # if it's used as `@attrs` but ``None`` if used as `@attrs()`.
+    if maybe_cls is None:
+        return wrap
+    else:
+        return wrap(maybe_cls)
+
+
+mutable = define
+frozen = partial(define, frozen=True, on_setattr=None)
+
+
+def field(
+    *,
+    default=NOTHING,
+    validator=None,
+    repr=True,
+    hash=None,
+    init=True,
+    metadata=None,
+    converter=None,
+    factory=None,
+    kw_only=False,
+    eq=None,
+    order=None,
+    on_setattr=None,
+):
+    """
+    Identical to `attr.ib`, except keyword-only and with some arguments
+    removed.
+
+    .. versionadded:: 20.1.0
+    """
+    return attrib(
+        default=default,
+        validator=validator,
+        repr=repr,
+        hash=hash,
+        init=init,
+        metadata=metadata,
+        converter=converter,
+        factory=factory,
+        kw_only=kw_only,
+        eq=eq,
+        order=order,
+        on_setattr=on_setattr,
+    )
diff --git a/poetry/core/_vendor/attr/converters.py b/poetry/core/_vendor/attr/converters.py
index 859289784..715ce1785 100644
--- a/poetry/core/_vendor/attr/converters.py
+++ b/poetry/core/_vendor/attr/converters.py
@@ -4,7 +4,14 @@
 
 from __future__ import absolute_import, division, print_function
 
-from ._make import NOTHING, Factory
+from ._make import NOTHING, Factory, pipe
+
+
+__all__ = [
+    "pipe",
+    "optional",
+    "default_if_none",
+]
 
 
 def optional(converter):
diff --git a/poetry/core/_vendor/attr/exceptions.py b/poetry/core/_vendor/attr/exceptions.py
index d1b76185c..fcd89106f 100644
--- a/poetry/core/_vendor/attr/exceptions.py
+++ b/poetry/core/_vendor/attr/exceptions.py
@@ -1,20 +1,37 @@
 from __future__ import absolute_import, division, print_function
 
 
-class FrozenInstanceError(AttributeError):
+class FrozenError(AttributeError):
     """
-    A frozen/immutable instance has been attempted to be modified.
+    A frozen/immutable instance or attribute haave been attempted to be
+    modified.
 
     It mirrors the behavior of ``namedtuples`` by using the same error message
     and subclassing `AttributeError`.
 
-    .. versionadded:: 16.1.0
+    .. versionadded:: 20.1.0
     """
 
     msg = "can't set attribute"
     args = [msg]
 
 
+class FrozenInstanceError(FrozenError):
+    """
+    A frozen instance has been attempted to be modified.
+
+    .. versionadded:: 16.1.0
+    """
+
+
+class FrozenAttributeError(FrozenError):
+    """
+    A frozen attribute has been attempted to be modified.
+
+    .. versionadded:: 20.1.0
+    """
+
+
 class AttrsAttributeNotFoundError(ValueError):
     """
     An ``attrs`` function couldn't find an attribute that the user asked for.
@@ -51,7 +68,8 @@ class UnannotatedAttributeError(RuntimeError):
 
 class PythonTooOldError(RuntimeError):
     """
-    An ``attrs`` feature requiring a more recent python version has been used.
+    It was attempted to use an ``attrs`` feature that requires a newer Python
+    version.
 
     .. versionadded:: 18.2.0
     """
diff --git a/poetry/core/_vendor/attr/setters.py b/poetry/core/_vendor/attr/setters.py
new file mode 100644
index 000000000..240014b3c
--- /dev/null
+++ b/poetry/core/_vendor/attr/setters.py
@@ -0,0 +1,77 @@
+"""
+Commonly used hooks for on_setattr.
+"""
+
+from __future__ import absolute_import, division, print_function
+
+from . import _config
+from .exceptions import FrozenAttributeError
+
+
+def pipe(*setters):
+    """
+    Run all *setters* and return the return value of the last one.
+
+    .. versionadded:: 20.1.0
+    """
+
+    def wrapped_pipe(instance, attrib, new_value):
+        rv = new_value
+
+        for setter in setters:
+            rv = setter(instance, attrib, rv)
+
+        return rv
+
+    return wrapped_pipe
+
+
+def frozen(_, __, ___):
+    """
+    Prevent an attribute to be modified.
+
+    .. versionadded:: 20.1.0
+    """
+    raise FrozenAttributeError()
+
+
+def validate(instance, attrib, new_value):
+    """
+    Run *attrib*'s validator on *new_value* if it has one.
+
+    .. versionadded:: 20.1.0
+    """
+    if _config._run_validators is False:
+        return new_value
+
+    v = attrib.validator
+    if not v:
+        return new_value
+
+    v(instance, attrib, new_value)
+
+    return new_value
+
+
+def convert(instance, attrib, new_value):
+    """
+    Run *attrib*'s converter -- if it has one --  on *new_value* and return the
+    result.
+
+    .. versionadded:: 20.1.0
+    """
+    c = attrib.converter
+    if c:
+        return c(new_value)
+
+    return new_value
+
+
+NO_OP = object()
+"""
+Sentinel for disabling class-wide *on_setattr* hooks for certain attributes.
+
+Does not work in `pipe` or within lists.
+
+.. versionadded:: 20.1.0
+"""
diff --git a/poetry/core/_vendor/attr/validators.py b/poetry/core/_vendor/attr/validators.py
index 839d310c3..b9a73054e 100644
--- a/poetry/core/_vendor/attr/validators.py
+++ b/poetry/core/_vendor/attr/validators.py
@@ -67,7 +67,7 @@ def instance_of(type):
     return _InstanceOfValidator(type)
 
 
-@attrs(repr=False, frozen=True)
+@attrs(repr=False, frozen=True, slots=True)
 class _MatchesReValidator(object):
     regex = attrib()
     flags = attrib()
@@ -171,7 +171,8 @@ def provides(interface):
     performed using ``interface.providedBy(value)`` (see `zope.interface
     <https://zopeinterface.readthedocs.io/en/latest/>`_).
 
-    :param zope.interface.Interface interface: The interface to check for.
+    :param interface: The interface to check for.
+    :type interface: ``zope.interface.Interface``
 
     :raises TypeError: With a human readable error message, the attribute
         (of type `attr.Attribute`), the expected interface, and the
diff --git a/poetry/core/_vendor/packaging/__about__.py b/poetry/core/_vendor/packaging/__about__.py
index 4d998578d..2d39193b0 100644
--- a/poetry/core/_vendor/packaging/__about__.py
+++ b/poetry/core/_vendor/packaging/__about__.py
@@ -18,10 +18,10 @@
 __summary__ = "Core utilities for Python packages"
 __uri__ = "https://github.com/pypa/packaging"
 
-__version__ = "20.4"
+__version__ = "20.8"
 
 __author__ = "Donald Stufft and individual contributors"
 __email__ = "donald@stufft.io"
 
 __license__ = "BSD-2-Clause or Apache-2.0"
-__copyright__ = "Copyright 2014-2019 %s" % __author__
+__copyright__ = "2014-2019 %s" % __author__
diff --git a/poetry/core/_vendor/packaging/requirements.py b/poetry/core/_vendor/packaging/requirements.py
index 91f81ede0..5ba8daf28 100644
--- a/poetry/core/_vendor/packaging/requirements.py
+++ b/poetry/core/_vendor/packaging/requirements.py
@@ -5,18 +5,24 @@
 
 import string
 import re
+import sys
 
 from pyparsing import stringStart, stringEnd, originalTextFor, ParseException
 from pyparsing import ZeroOrMore, Word, Optional, Regex, Combine
 from pyparsing import Literal as L  # noqa
-from six.moves.urllib import parse as urlparse
 
 from ._typing import TYPE_CHECKING
 from .markers import MARKER_EXPR, Marker
 from .specifiers import LegacySpecifier, Specifier, SpecifierSet
 
+if sys.version_info[0] >= 3:
+    from urllib import parse as urlparse  # pragma: no cover
+else:  # pragma: no cover
+    import urlparse
+
+
 if TYPE_CHECKING:  # pragma: no cover
-    from typing import List
+    from typing import List, Optional as TOptional, Set
 
 
 class InvalidRequirement(ValueError):
@@ -103,7 +109,7 @@ def __init__(self, requirement_string):
                 )
             )
 
-        self.name = req.name
+        self.name = req.name  # type: str
         if req.url:
             parsed_url = urlparse.urlparse(req.url)
             if parsed_url.scheme == "file":
@@ -113,12 +119,12 @@ def __init__(self, requirement_string):
                 not parsed_url.scheme and not parsed_url.netloc
             ):
                 raise InvalidRequirement("Invalid URL: {0}".format(req.url))
-            self.url = req.url
+            self.url = req.url  # type: TOptional[str]
         else:
             self.url = None
-        self.extras = set(req.extras.asList() if req.extras else [])
-        self.specifier = SpecifierSet(req.specifier)
-        self.marker = req.marker if req.marker else None
+        self.extras = set(req.extras.asList() if req.extras else [])  # type: Set[str]
+        self.specifier = SpecifierSet(req.specifier)  # type: SpecifierSet
+        self.marker = req.marker if req.marker else None  # type: TOptional[Marker]
 
     def __str__(self):
         # type: () -> str
diff --git a/poetry/core/_vendor/packaging/specifiers.py b/poetry/core/_vendor/packaging/specifiers.py
index fe09bb1db..a42cbfef3 100644
--- a/poetry/core/_vendor/packaging/specifiers.py
+++ b/poetry/core/_vendor/packaging/specifiers.py
@@ -7,6 +7,7 @@
 import functools
 import itertools
 import re
+import warnings
 
 from ._compat import string_types, with_metaclass
 from ._typing import TYPE_CHECKING
@@ -14,17 +15,7 @@
 from .version import Version, LegacyVersion, parse
 
 if TYPE_CHECKING:  # pragma: no cover
-    from typing import (
-        List,
-        Dict,
-        Union,
-        Iterable,
-        Iterator,
-        Optional,
-        Callable,
-        Tuple,
-        FrozenSet,
-    )
+    from typing import List, Dict, Union, Iterable, Iterator, Optional, Callable, Tuple
 
     ParsedVersion = Union[Version, LegacyVersion]
     UnparsedVersion = Union[Version, LegacyVersion, str]
@@ -285,6 +276,16 @@ class LegacySpecifier(_IndividualSpecifier):
         ">": "greater_than",
     }
 
+    def __init__(self, spec="", prereleases=None):
+        # type: (str, Optional[bool]) -> None
+        super(LegacySpecifier, self).__init__(spec, prereleases)
+
+        warnings.warn(
+            "Creating a LegacyVersion has been deprecated and will be "
+            "removed in the next major release",
+            DeprecationWarning,
+        )
+
     def _coerce_version(self, version):
         # type: (Union[ParsedVersion, str]) -> LegacyVersion
         if not isinstance(version, LegacyVersion):
@@ -317,7 +318,7 @@ def _compare_greater_than(self, prospective, spec):
 
 
 def _require_version_compare(
-    fn  # type: (Callable[[Specifier, ParsedVersion, str], bool])
+    fn,  # type: (Callable[[Specifier, ParsedVersion, str], bool])
 ):
     # type: (...) -> Callable[[Specifier, ParsedVersion, str], bool]
     @functools.wraps(fn)
@@ -750,7 +751,7 @@ def __len__(self):
         return len(self._specs)
 
     def __iter__(self):
-        # type: () -> Iterator[FrozenSet[_IndividualSpecifier]]
+        # type: () -> Iterator[_IndividualSpecifier]
         return iter(self._specs)
 
     @property
diff --git a/poetry/core/_vendor/packaging/tags.py b/poetry/core/_vendor/packaging/tags.py
index 9064910b8..13798e38b 100644
--- a/poetry/core/_vendor/packaging/tags.py
+++ b/poetry/core/_vendor/packaging/tags.py
@@ -13,6 +13,7 @@
 
     EXTENSION_SUFFIXES = [x[0] for x in imp.get_suffixes()]
     del imp
+import collections
 import logging
 import os
 import platform
@@ -57,6 +58,24 @@
 _32_BIT_INTERPRETER = sys.maxsize <= 2 ** 32
 
 
+_LEGACY_MANYLINUX_MAP = {
+    # CentOS 7 w/ glibc 2.17 (PEP 599)
+    (2, 17): "manylinux2014",
+    # CentOS 6 w/ glibc 2.12 (PEP 571)
+    (2, 12): "manylinux2010",
+    # CentOS 5 w/ glibc 2.5 (PEP 513)
+    (2, 5): "manylinux1",
+}
+
+# If glibc ever changes its major version, we need to know what the last
+# minor version was, so we can build the complete list of all versions.
+# For now, guess what the highest minor version might be, assume it will
+# be 50 for testing. Once this actually happens, update the dictionary
+# with the actual value.
+_LAST_GLIBC_MINOR = collections.defaultdict(lambda: 50)  # type: Dict[int, int]
+glibcVersion = collections.namedtuple("Version", ["major", "minor"])
+
+
 class Tag(object):
     """
     A representation of the tag triple for a wheel.
@@ -65,13 +84,19 @@ class Tag(object):
     is also supported.
     """
 
-    __slots__ = ["_interpreter", "_abi", "_platform"]
+    __slots__ = ["_interpreter", "_abi", "_platform", "_hash"]
 
     def __init__(self, interpreter, abi, platform):
         # type: (str, str, str) -> None
         self._interpreter = interpreter.lower()
         self._abi = abi.lower()
         self._platform = platform.lower()
+        # The __hash__ of every single element in a Set[Tag] will be evaluated each time
+        # that a set calls its `.disjoint()` method, which may be called hundreds of
+        # times when scanning a page of links for packages with tags matching that
+        # Set[Tag]. Pre-computing the value here produces significant speedups for
+        # downstream consumers.
+        self._hash = hash((self._interpreter, self._abi, self._platform))
 
     @property
     def interpreter(self):
@@ -101,7 +126,7 @@ def __eq__(self, other):
 
     def __hash__(self):
         # type: () -> int
-        return hash((self._interpreter, self._abi, self._platform))
+        return self._hash
 
     def __str__(self):
         # type: () -> str
@@ -382,7 +407,12 @@ def _mac_binary_formats(version, cpu_arch):
             return []
         formats.extend(["fat32", "fat"])
 
-    formats.append("universal")
+    if cpu_arch in {"arm64", "x86_64"}:
+        formats.append("universal2")
+
+    if cpu_arch in {"x86_64", "i386", "ppc64", "ppc", "intel"}:
+        formats.append("universal")
+
     return formats
 
 
@@ -405,30 +435,73 @@ def mac_platforms(version=None, arch=None):
         arch = _mac_arch(cpu_arch)
     else:
         arch = arch
-    for minor_version in range(version[1], -1, -1):
-        compat_version = version[0], minor_version
-        binary_formats = _mac_binary_formats(compat_version, arch)
-        for binary_format in binary_formats:
-            yield "macosx_{major}_{minor}_{binary_format}".format(
-                major=compat_version[0],
-                minor=compat_version[1],
-                binary_format=binary_format,
-            )
 
+    if (10, 0) <= version and version < (11, 0):
+        # Prior to Mac OS 11, each yearly release of Mac OS bumped the
+        # "minor" version number.  The major version was always 10.
+        for minor_version in range(version[1], -1, -1):
+            compat_version = 10, minor_version
+            binary_formats = _mac_binary_formats(compat_version, arch)
+            for binary_format in binary_formats:
+                yield "macosx_{major}_{minor}_{binary_format}".format(
+                    major=10, minor=minor_version, binary_format=binary_format
+                )
+
+    if version >= (11, 0):
+        # Starting with Mac OS 11, each yearly release bumps the major version
+        # number.   The minor versions are now the midyear updates.
+        for major_version in range(version[0], 10, -1):
+            compat_version = major_version, 0
+            binary_formats = _mac_binary_formats(compat_version, arch)
+            for binary_format in binary_formats:
+                yield "macosx_{major}_{minor}_{binary_format}".format(
+                    major=major_version, minor=0, binary_format=binary_format
+                )
 
-# From PEP 513.
-def _is_manylinux_compatible(name, glibc_version):
-    # type: (str, GlibcVersion) -> bool
+    if version >= (11, 0) and arch == "x86_64":
+        # Mac OS 11 on x86_64 is compatible with binaries from previous releases.
+        # Arm64 support was introduced in 11.0, so no Arm binaries from previous
+        # releases exist.
+        for minor_version in range(16, 3, -1):
+            compat_version = 10, minor_version
+            binary_formats = _mac_binary_formats(compat_version, arch)
+            for binary_format in binary_formats:
+                yield "macosx_{major}_{minor}_{binary_format}".format(
+                    major=compat_version[0],
+                    minor=compat_version[1],
+                    binary_format=binary_format,
+                )
+
+
+# From PEP 513, PEP 600
+def _is_manylinux_compatible(name, arch, glibc_version):
+    # type: (str, str, GlibcVersion) -> bool
+    sys_glibc = _get_glibc_version()
+    if sys_glibc < glibc_version:
+        return False
     # Check for presence of _manylinux module.
     try:
         import _manylinux  # noqa
-
-        return bool(getattr(_manylinux, name + "_compatible"))
-    except (ImportError, AttributeError):
-        # Fall through to heuristic check below.
+    except ImportError:
         pass
-
-    return _have_compatible_glibc(*glibc_version)
+    else:
+        if hasattr(_manylinux, "manylinux_compatible"):
+            result = _manylinux.manylinux_compatible(
+                glibc_version[0], glibc_version[1], arch
+            )
+            if result is not None:
+                return bool(result)
+        else:
+            if glibc_version == (2, 5):
+                if hasattr(_manylinux, "manylinux1_compatible"):
+                    return bool(_manylinux.manylinux1_compatible)
+            if glibc_version == (2, 12):
+                if hasattr(_manylinux, "manylinux2010_compatible"):
+                    return bool(_manylinux.manylinux2010_compatible)
+            if glibc_version == (2, 17):
+                if hasattr(_manylinux, "manylinux2014_compatible"):
+                    return bool(_manylinux.manylinux2014_compatible)
+    return True
 
 
 def _glibc_version_string():
@@ -474,8 +547,20 @@ def _glibc_version_string_ctypes():
     # main program". This way we can let the linker do the work to figure out
     # which libc our process is actually using.
     #
-    # Note: typeshed is wrong here so we are ignoring this line.
-    process_namespace = ctypes.CDLL(None)  # type: ignore
+    # We must also handle the special case where the executable is not a
+    # dynamically linked executable. This can occur when using musl libc,
+    # for example. In this situation, dlopen() will error, leading to an
+    # OSError. Interestingly, at least in the case of musl, there is no
+    # errno set on the OSError. The single string argument used to construct
+    # OSError comes from libc itself and is therefore not portable to
+    # hard code here. In any case, failure to call dlopen() means we
+    # can proceed, so we bail on our attempt.
+    try:
+        # Note: typeshed is wrong here so we are ignoring this line.
+        process_namespace = ctypes.CDLL(None)  # type: ignore
+    except OSError:
+        return None
+
     try:
         gnu_get_libc_version = process_namespace.gnu_get_libc_version
     except AttributeError:
@@ -493,10 +578,9 @@ def _glibc_version_string_ctypes():
     return version_str
 
 
-# Separated out from have_compatible_glibc for easier unit testing.
-def _check_glibc_version(version_str, required_major, minimum_minor):
-    # type: (str, int, int) -> bool
-    # Parse string and check against requested version.
+def _parse_glibc_version(version_str):
+    # type: (str) -> Tuple[int, int]
+    # Parse glibc version.
     #
     # We use a regexp instead of str.split because we want to discard any
     # random junk that might come after the minor version -- this might happen
@@ -509,19 +593,23 @@ def _check_glibc_version(version_str, required_major, minimum_minor):
             " got: %s" % version_str,
             RuntimeWarning,
         )
-        return False
-    return (
-        int(m.group("major")) == required_major
-        and int(m.group("minor")) >= minimum_minor
-    )
+        return -1, -1
+    return (int(m.group("major")), int(m.group("minor")))
+
 
+_glibc_version = []  #  type: List[Tuple[int, int]]
 
-def _have_compatible_glibc(required_major, minimum_minor):
-    # type: (int, int) -> bool
+
+def _get_glibc_version():
+    # type: () -> Tuple[int, int]
+    if _glibc_version:
+        return _glibc_version[0]
     version_str = _glibc_version_string()
     if version_str is None:
-        return False
-    return _check_glibc_version(version_str, required_major, minimum_minor)
+        _glibc_version.append((-1, -1))
+    else:
+        _glibc_version.append(_parse_glibc_version(version_str))
+    return _glibc_version[0]
 
 
 # Python does not provide platform information at sufficient granularity to
@@ -639,7 +727,42 @@ def _have_compatible_manylinux_abi(arch):
         return _is_linux_armhf()
     if arch == "i686":
         return _is_linux_i686()
-    return True
+    return arch in {"x86_64", "aarch64", "ppc64", "ppc64le", "s390x"}
+
+
+def _manylinux_tags(linux, arch):
+    # type: (str, str) -> Iterator[str]
+    # Oldest glibc to be supported regardless of architecture is (2, 17).
+    too_old_glibc2 = glibcVersion(2, 16)
+    if arch in {"x86_64", "i686"}:
+        # On x86/i686 also oldest glibc to be supported is (2, 5).
+        too_old_glibc2 = glibcVersion(2, 4)
+    current_glibc = glibcVersion(*_get_glibc_version())
+    glibc_max_list = [current_glibc]
+    # We can assume compatibility across glibc major versions.
+    # https://sourceware.org/bugzilla/show_bug.cgi?id=24636
+    #
+    # Build a list of maximum glibc versions so that we can
+    # output the canonical list of all glibc from current_glibc
+    # down to too_old_glibc2, including all intermediary versions.
+    for glibc_major in range(current_glibc.major - 1, 1, -1):
+        glibc_max_list.append(glibcVersion(glibc_major, _LAST_GLIBC_MINOR[glibc_major]))
+    for glibc_max in glibc_max_list:
+        if glibc_max.major == too_old_glibc2.major:
+            min_minor = too_old_glibc2.minor
+        else:
+            # For other glibc major versions oldest supported is (x, 0).
+            min_minor = -1
+        for glibc_minor in range(glibc_max.minor, min_minor, -1):
+            glibc_version = (glibc_max.major, glibc_minor)
+            tag = "manylinux_{}_{}".format(*glibc_version)
+            if _is_manylinux_compatible(tag, arch, glibc_version):
+                yield linux.replace("linux", tag)
+            # Handle the legacy manylinux1, manylinux2010, manylinux2014 tags.
+            if glibc_version in _LEGACY_MANYLINUX_MAP:
+                legacy_tag = _LEGACY_MANYLINUX_MAP[glibc_version]
+                if _is_manylinux_compatible(legacy_tag, arch, glibc_version):
+                    yield linux.replace("linux", legacy_tag)
 
 
 def _linux_platforms(is_32bit=_32_BIT_INTERPRETER):
@@ -650,28 +773,10 @@ def _linux_platforms(is_32bit=_32_BIT_INTERPRETER):
             linux = "linux_i686"
         elif linux == "linux_aarch64":
             linux = "linux_armv7l"
-    manylinux_support = []
     _, arch = linux.split("_", 1)
     if _have_compatible_manylinux_abi(arch):
-        if arch in {"x86_64", "i686", "aarch64", "armv7l", "ppc64", "ppc64le", "s390x"}:
-            manylinux_support.append(
-                ("manylinux2014", (2, 17))
-            )  # CentOS 7 w/ glibc 2.17 (PEP 599)
-        if arch in {"x86_64", "i686"}:
-            manylinux_support.append(
-                ("manylinux2010", (2, 12))
-            )  # CentOS 6 w/ glibc 2.12 (PEP 571)
-            manylinux_support.append(
-                ("manylinux1", (2, 5))
-            )  # CentOS 5 w/ glibc 2.5 (PEP 513)
-    manylinux_support_iter = iter(manylinux_support)
-    for name, glibc_version in manylinux_support_iter:
-        if _is_manylinux_compatible(name, glibc_version):
-            yield linux.replace("linux", name)
-            break
-    # Support for a later manylinux implies support for an earlier version.
-    for name, _ in manylinux_support_iter:
-        yield linux.replace("linux", name)
+        for tag in _manylinux_tags(linux, arch):
+            yield tag
     yield linux
 
 
@@ -722,11 +827,7 @@ def interpreter_version(**kwargs):
 
 def _version_nodot(version):
     # type: (PythonVersion) -> str
-    if any(v >= 10 for v in version):
-        sep = "_"
-    else:
-        sep = ""
-    return sep.join(map(str, version))
+    return "".join(map(str, version))
 
 
 def sys_tags(**kwargs):
diff --git a/poetry/core/_vendor/packaging/utils.py b/poetry/core/_vendor/packaging/utils.py
index 19579c1a0..92c7b00b7 100644
--- a/poetry/core/_vendor/packaging/utils.py
+++ b/poetry/core/_vendor/packaging/utils.py
@@ -12,6 +12,8 @@
     from typing import NewType, Union
 
     NormalizedName = NewType("NormalizedName", str)
+else:
+    NormalizedName = str
 
 _canonicalize_regex = re.compile(r"[-_.]+")
 
@@ -23,18 +25,18 @@ def canonicalize_name(name):
     return cast("NormalizedName", value)
 
 
-def canonicalize_version(_version):
-    # type: (str) -> Union[Version, str]
+def canonicalize_version(version):
+    # type: (Union[Version, str]) -> Union[Version, str]
     """
     This is very similar to Version.__str__, but has one subtle difference
     with the way it handles the release segment.
     """
-
-    try:
-        version = Version(_version)
-    except InvalidVersion:
-        # Legacy versions cannot be normalized
-        return _version
+    if not isinstance(version, Version):
+        try:
+            version = Version(version)
+        except InvalidVersion:
+            # Legacy versions cannot be normalized
+            return version
 
     parts = []
 
diff --git a/poetry/core/_vendor/packaging/version.py b/poetry/core/_vendor/packaging/version.py
index 00371e86a..517d91f24 100644
--- a/poetry/core/_vendor/packaging/version.py
+++ b/poetry/core/_vendor/packaging/version.py
@@ -6,6 +6,7 @@
 import collections
 import itertools
 import re
+import warnings
 
 from ._structures import Infinity, NegativeInfinity
 from ._typing import TYPE_CHECKING
@@ -71,36 +72,50 @@ def __hash__(self):
         # type: () -> int
         return hash(self._key)
 
+    # Please keep the duplicated `isinstance` check
+    # in the six comparisons hereunder
+    # unless you find a way to avoid adding overhead function calls.
     def __lt__(self, other):
         # type: (_BaseVersion) -> bool
-        return self._compare(other, lambda s, o: s < o)
+        if not isinstance(other, _BaseVersion):
+            return NotImplemented
+
+        return self._key < other._key
 
     def __le__(self, other):
         # type: (_BaseVersion) -> bool
-        return self._compare(other, lambda s, o: s <= o)
+        if not isinstance(other, _BaseVersion):
+            return NotImplemented
+
+        return self._key <= other._key
 
     def __eq__(self, other):
         # type: (object) -> bool
-        return self._compare(other, lambda s, o: s == o)
+        if not isinstance(other, _BaseVersion):
+            return NotImplemented
+
+        return self._key == other._key
 
     def __ge__(self, other):
         # type: (_BaseVersion) -> bool
-        return self._compare(other, lambda s, o: s >= o)
+        if not isinstance(other, _BaseVersion):
+            return NotImplemented
+
+        return self._key >= other._key
 
     def __gt__(self, other):
         # type: (_BaseVersion) -> bool
-        return self._compare(other, lambda s, o: s > o)
+        if not isinstance(other, _BaseVersion):
+            return NotImplemented
+
+        return self._key > other._key
 
     def __ne__(self, other):
         # type: (object) -> bool
-        return self._compare(other, lambda s, o: s != o)
-
-    def _compare(self, other, method):
-        # type: (object, VersionComparisonMethod) -> Union[bool, NotImplemented]
         if not isinstance(other, _BaseVersion):
             return NotImplemented
 
-        return method(self._key, other._key)
+        return self._key != other._key
 
 
 class LegacyVersion(_BaseVersion):
@@ -109,6 +124,12 @@ def __init__(self, version):
         self._version = str(version)
         self._key = _legacy_cmpkey(self._version)
 
+        warnings.warn(
+            "Creating a LegacyVersion has been deprecated and will be "
+            "removed in the next major release",
+            DeprecationWarning,
+        )
+
     def __str__(self):
         # type: () -> str
         return self._version
diff --git a/poetry/core/_vendor/pyrsistent/LICENCE.mit b/poetry/core/_vendor/pyrsistent/LICENSE.mit
similarity index 100%
rename from poetry/core/_vendor/pyrsistent/LICENCE.mit
rename to poetry/core/_vendor/pyrsistent/LICENSE.mit
diff --git a/poetry/core/_vendor/vendor.txt b/poetry/core/_vendor/vendor.txt
index a1c24ba96..6667b3d32 100644
--- a/poetry/core/_vendor/vendor.txt
+++ b/poetry/core/_vendor/vendor.txt
@@ -1,8 +1,9 @@
-attrs==19.3.0
+attrs==20.3.0; python_version >= "2.7" and python_full_version < "3.0.0" or python_full_version >= "3.4.0"
 jsonschema==3.2.0
 lark-parser==0.9.0
-packaging==20.4
-pyparsing==2.4.7
-pyrsistent==0.16.0
-six==1.15.0
-tomlkit==0.7.0
+packaging==20.8; (python_version >= "2.7" and python_full_version < "3.0.0") or (python_full_version >= "3.4.0")
+pyparsing==2.4.7; python_version >= "2.7" and python_full_version < "3.0.0" or python_full_version >= "3.4.0"
+pyrsistent==0.16.1; python_version >= "2.7"
+six==1.15.0; python_version >= "2.7" and python_full_version < "3.0.0" or python_full_version >= "3.3.0" and python_version >= "2.7"
+tomlkit==0.7.0; (python_version >= "2.7" and python_full_version < "3.0.0") or (python_full_version >= "3.5.0")
+typing-extensions==3.7.4.3; python_version >= "3.6" and python_version < "3.8"
diff --git a/pyproject.toml b/pyproject.toml
index 628cf484b..9948932ab 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -45,6 +45,7 @@ functools32 = {version = "^3.2.3-2", python = "~2.7"}
 
 [tool.poetry.dev-dependencies]
 pre-commit = "^1.10"
+pyrsistent = "^0.16.0"
 pytest = "^4.6"
 pytest-cov = "^2.8"
 pytest-mock = "^2.0"
@@ -113,7 +114,7 @@ appdirs = []
 
 
 [tool.vendoring.license.fallback-urls]
-pyrsistent = "https://raw.githubusercontent.com/tobgu/pyrsistent/master/LICENCE.mit"
+pyrsistent = "https://raw.githubusercontent.com/tobgu/pyrsistent/master/LICENSE.mit"
 
 [build-system]
 requires = []
diff --git a/vendors/poetry.lock b/vendors/poetry.lock
index 3e6414c96..b6f1d79ad 100644
--- a/vendors/poetry.lock
+++ b/vendors/poetry.lock
@@ -1,32 +1,33 @@
 [[package]]
 name = "attrs"
-version = "19.3.0"
+version = "20.3.0"
 description = "Classes Without Boilerplate"
 category = "main"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
 [package.extras]
-azure-pipelines = ["coverage", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "zope.interface", "pytest-azurepipelines"]
-dev = ["coverage", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "zope.interface", "sphinx", "pre-commit"]
-docs = ["sphinx", "zope.interface"]
-tests = ["coverage", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "zope.interface"]
+dev = ["coverage[toml] (>=5.0.2)", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "zope.interface", "furo", "sphinx", "pre-commit"]
+docs = ["furo", "sphinx", "zope.interface"]
+tests = ["coverage[toml] (>=5.0.2)", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "zope.interface"]
+tests_no_zope = ["coverage[toml] (>=5.0.2)", "hypothesis", "pympler", "pytest (>=4.3.0)", "six"]
 
 [[package]]
 name = "importlib-metadata"
-version = "1.7.0"
+version = "3.4.0"
 description = "Read metadata from Python packages"
 category = "main"
 optional = false
-python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,>=2.7"
-
-[package.extras]
-docs = ["sphinx", "rst.linker"]
-testing = ["packaging", "pep517", "importlib-resources (>=1.3)"]
+python-versions = ">=3.6"
 
 [package.dependencies]
+typing-extensions = {version = ">=3.6.4", markers = "python_version < \"3.8\""}
 zipp = ">=0.5"
 
+[package.extras]
+docs = ["sphinx", "jaraco.packaging (>=8.2)", "rst.linker (>=1.9)"]
+testing = ["pytest (>=3.5,!=3.7.3)", "pytest-checkdocs (>=1.2.3)", "pytest-flake8", "pytest-cov", "pytest-enabler", "packaging", "pep517", "pyfakefs", "flufl.flake8", "pytest-black (>=0.3.7)", "pytest-mypy", "importlib-resources (>=1.3)"]
+
 [[package]]
 name = "jsonschema"
 version = "3.2.0"
@@ -35,15 +36,15 @@ category = "main"
 optional = false
 python-versions = "*"
 
-[package.extras]
-format = ["idna", "jsonpointer (>1.13)", "rfc3987", "strict-rfc3339", "webcolors"]
-format_nongpl = ["idna", "jsonpointer (>1.13)", "webcolors", "rfc3986-validator (>0.1.0)", "rfc3339-validator"]
-
 [package.dependencies]
 attrs = ">=17.4.0"
+importlib-metadata = {version = "*", markers = "python_version < \"3.8\""}
 pyrsistent = ">=0.14.0"
 six = ">=1.11.0"
-importlib-metadata = {version = "*", markers = "python_version < \"3.8\""}
+
+[package.extras]
+format = ["idna", "jsonpointer (>1.13)", "rfc3987", "strict-rfc3339", "webcolors"]
+format_nongpl = ["idna", "jsonpointer (>1.13)", "webcolors", "rfc3986-validator (>0.1.0)", "rfc3339-validator"]
 
 [[package]]
 name = "lark-parser"
@@ -58,7 +59,7 @@ regex = ["regex"]
 
 [[package]]
 name = "packaging"
-version = "20.4"
+version = "20.8"
 description = "Core utilities for Python packages"
 category = "main"
 optional = false
@@ -66,7 +67,6 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
 [package.dependencies]
 pyparsing = ">=2.0.2"
-six = "*"
 
 [[package]]
 name = "pyparsing"
@@ -78,11 +78,11 @@ python-versions = ">=2.6, !=3.0.*, !=3.1.*, !=3.2.*"
 
 [[package]]
 name = "pyrsistent"
-version = "0.16.0"
+version = "0.16.1"
 description = "Persistent/Functional/Immutable data structures"
 category = "main"
 optional = false
-python-versions = "*"
+python-versions = ">=2.7"
 
 [package.dependencies]
 six = "*"
@@ -103,9 +103,17 @@ category = "main"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
 
+[[package]]
+name = "typing-extensions"
+version = "3.7.4.3"
+description = "Backported and Experimental Type Hints for Python 3.5+"
+category = "main"
+optional = false
+python-versions = "*"
+
 [[package]]
 name = "zipp"
-version = "3.1.0"
+version = "3.4.0"
 description = "Backport of pathlib-compatible object wrapper for zip files"
 category = "main"
 optional = false
@@ -113,21 +121,21 @@ python-versions = ">=3.6"
 
 [package.extras]
 docs = ["sphinx", "jaraco.packaging (>=3.2)", "rst.linker (>=1.9)"]
-testing = ["jaraco.itertools", "func-timeout"]
+testing = ["pytest (>=3.5,!=3.7.3)", "pytest-checkdocs (>=1.2.3)", "pytest-flake8", "pytest-cov", "jaraco.test (>=3.2.0)", "jaraco.itertools", "func-timeout", "pytest-black (>=0.3.7)", "pytest-mypy"]
 
 [metadata]
 lock-version = "1.1"
 python-versions = "^3.6"
-content-hash = "cf00c694f857a9caf5610127561a5cd3d571db9420f4e4ebb6c787cefa32af31"
+content-hash = "f9770372f7c711e4c2941e30c28b26a58a8c76aae88ddc1e3ea8e175fcc6a534"
 
 [metadata.files]
 attrs = [
-    {file = "attrs-19.3.0-py2.py3-none-any.whl", hash = "sha256:08a96c641c3a74e44eb59afb61a24f2cb9f4d7188748e76ba4bb5edfa3cb7d1c"},
-    {file = "attrs-19.3.0.tar.gz", hash = "sha256:f7b7ce16570fe9965acd6d30101a28f62fb4a7f9e926b3bbc9b61f8b04247e72"},
+    {file = "attrs-20.3.0-py2.py3-none-any.whl", hash = "sha256:31b2eced602aa8423c2aea9c76a724617ed67cf9513173fd3a4f03e3a929c7e6"},
+    {file = "attrs-20.3.0.tar.gz", hash = "sha256:832aa3cde19744e49938b91fea06d69ecb9e649c93ba974535d08ad92164f700"},
 ]
 importlib-metadata = [
-    {file = "importlib_metadata-1.7.0-py2.py3-none-any.whl", hash = "sha256:dc15b2969b4ce36305c51eebe62d418ac7791e9a157911d58bfb1f9ccd8e2070"},
-    {file = "importlib_metadata-1.7.0.tar.gz", hash = "sha256:90bb658cdbbf6d1735b6341ce708fc7024a3e14e99ffdc5783edea9f9b077f83"},
+    {file = "importlib_metadata-3.4.0-py3-none-any.whl", hash = "sha256:ace61d5fc652dc280e7b6b4ff732a9c2d40db2c0f92bc6cb74e07b73d53a1771"},
+    {file = "importlib_metadata-3.4.0.tar.gz", hash = "sha256:fa5daa4477a7414ae34e95942e4dd07f62adf589143c875c133c1e53c4eff38d"},
 ]
 jsonschema = [
     {file = "jsonschema-3.2.0-py2.py3-none-any.whl", hash = "sha256:4e5b3cf8216f577bee9ce139cbe72eca3ea4f292ec60928ff24758ce626cd163"},
@@ -137,15 +145,15 @@ lark-parser = [
     {file = "lark-parser-0.9.0.tar.gz", hash = "sha256:9e7589365d6b6de1cca40b0eaec31104a3fb96a37a11a9dfd5098e95b50aa6cd"},
 ]
 packaging = [
-    {file = "packaging-20.4-py2.py3-none-any.whl", hash = "sha256:998416ba6962ae7fbd6596850b80e17859a5753ba17c32284f67bfff33784181"},
-    {file = "packaging-20.4.tar.gz", hash = "sha256:4357f74f47b9c12db93624a82154e9b120fa8293699949152b22065d556079f8"},
+    {file = "packaging-20.8-py2.py3-none-any.whl", hash = "sha256:24e0da08660a87484d1602c30bb4902d74816b6985b93de36926f5bc95741858"},
+    {file = "packaging-20.8.tar.gz", hash = "sha256:78598185a7008a470d64526a8059de9aaa449238f280fc9eb6b13ba6c4109093"},
 ]
 pyparsing = [
     {file = "pyparsing-2.4.7-py2.py3-none-any.whl", hash = "sha256:ef9d7589ef3c200abe66653d3f1ab1033c3c419ae9b9bdb1240a85b024efc88b"},
     {file = "pyparsing-2.4.7.tar.gz", hash = "sha256:c203ec8783bf771a155b207279b9bccb8dea02d8f0c9e5f8ead507bc3246ecc1"},
 ]
 pyrsistent = [
-    {file = "pyrsistent-0.16.0.tar.gz", hash = "sha256:28669905fe725965daa16184933676547c5bb40a5153055a8dee2a4bd7933ad3"},
+    {file = "pyrsistent-0.16.1.tar.gz", hash = "sha256:aa2ae1c2e496f4d6777f869ea5de7166a8ccb9c2e06ebcf6c7ff1b670c98c5ef"},
 ]
 six = [
     {file = "six-1.15.0-py2.py3-none-any.whl", hash = "sha256:8b74bedcbbbaca38ff6d7491d76f2b06b3592611af620f8426e82dddb04a5ced"},
@@ -155,7 +163,12 @@ tomlkit = [
     {file = "tomlkit-0.7.0-py2.py3-none-any.whl", hash = "sha256:6babbd33b17d5c9691896b0e68159215a9387ebfa938aa3ac42f4a4beeb2b831"},
     {file = "tomlkit-0.7.0.tar.gz", hash = "sha256:ac57f29693fab3e309ea789252fcce3061e19110085aa31af5446ca749325618"},
 ]
+typing-extensions = [
+    {file = "typing_extensions-3.7.4.3-py2-none-any.whl", hash = "sha256:dafc7639cde7f1b6e1acc0f457842a83e722ccca8eef5270af2d74792619a89f"},
+    {file = "typing_extensions-3.7.4.3-py3-none-any.whl", hash = "sha256:7cb407020f00f7bfc3cb3e7881628838e69d8f3fcab2f64742a5e76b2f841918"},
+    {file = "typing_extensions-3.7.4.3.tar.gz", hash = "sha256:99d4073b617d30288f569d3f13d2bd7548c3a7e4c8de87db09a9d29bb3a4a60c"},
+]
 zipp = [
-    {file = "zipp-3.1.0-py3-none-any.whl", hash = "sha256:aa36550ff0c0b7ef7fa639055d797116ee891440eac1a56f378e2d3179e0320b"},
-    {file = "zipp-3.1.0.tar.gz", hash = "sha256:c599e4d75c98f6798c509911d08a22e6c021d074469042177c8c86fb92eefd96"},
+    {file = "zipp-3.4.0-py3-none-any.whl", hash = "sha256:102c24ef8f171fd729d46599845e95c7ab894a4cf45f5de11a44cc7444fb1108"},
+    {file = "zipp-3.4.0.tar.gz", hash = "sha256:ed5eee1974372595f9e416cc7bbeeb12335201d8081ca8a0743c954d4446e5cb"},
 ]
diff --git a/vendors/pyproject.toml b/vendors/pyproject.toml
index 69ccbd0f0..ff8020cd5 100644
--- a/vendors/pyproject.toml
+++ b/vendors/pyproject.toml
@@ -23,5 +23,6 @@ python = "^3.6"
 
 jsonschema = "^3.2.0"
 lark-parser = "^0.9.0"
-packaging = "^20.1"
+packaging = "^20.8"
+pyrsistent = "^0.16.0"
 tomlkit = ">=0.7.0,<1.0.0"