Document and start testing Python 3.9 support

At first use it for GitHub Action for linting.

.github/workflows/linting.yml

@@ -23,10 +23,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python 3.8
-      uses: actions/setup-python@v1
+    - name: Set up Python 3.9
+      uses: actions/setup-python@v2
       with:
-        python-version: 3.8
+        python-version: 3.9
 
     # Setup Caching
     - name: pip cache

news/8971.feature.rst

@@ -0,0 +1 @@
+Support Python 3.9.

noxfile.py

@@ -69,7 +69,7 @@ def should_update_common_wheels():
 #   completely to nox for all our automation. Contributors should prefer using
 #   `tox -e ...` until this note is removed.
 # -----------------------------------------------------------------------------
-@nox.session(python=["2.7", "3.5", "3.6", "3.7", "3.8", "pypy", "pypy3"])
+@nox.session(python=["2.7", "3.5", "3.6", "3.7", "3.8", "3.9", "pypy", "pypy3"])
 def test(session):
     # Get the common wheels.
     if should_update_common_wheels():

setup.py

@@ -47,6 +47,7 @@ def get_version(rel_path):
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
         "Programming Language :: Python :: Implementation :: CPython",
         "Programming Language :: Python :: Implementation :: PyPy",
     ],

tox.ini

@@ -2,7 +2,7 @@
 minversion = 3.4.0
 envlist =
     docs, packaging, lint, vendoring,
-    py27, py35, py36, py37, py38, pypy, pypy3
+    py27, py35, py36, py37, py38, py39, pypy, pypy3
 
 [helpers]
 # Wrapper for calls to pip that make sure the version being used is the
@@ -49,7 +49,7 @@ setenv =
 [testenv:docs]
 # Don't skip install here since pip_sphinxext uses pip's internals.
 deps = -r{toxinidir}/tools/requirements/docs.txt
-basepython = python3.8
+basepython = python3
 commands =
     sphinx-build -W -d {envtmpdir}/doctrees/html -b html docs/html docs/build/html
     # Having the conf.py in the docs/html is weird but needed because we
@@ -66,11 +66,11 @@ commands =
    pre-commit run [] --all-files --show-diff-on-failure
 
 [testenv:vendoring]
-basepython = python3.8
+basepython = python3
 skip_install = True
 commands_pre =
 deps =
-    vendoring==0.2.2
+    vendoring>=0.3.3
     # Required, otherwise we interpret --no-binary :all: as
     # "do not build wheels", which fails for PEP 517 requirements
     pip>=19.3.1