env: downgrade importlib-metadata to fix pipeline on python3.5

.travis.yml

@@ -63,12 +63,6 @@ matrix:
 install:
 - pip install pipenv
 - pipenv sync --dev
-# ModuleNotFoundError: No module named 'importlib_metadata'
-# https://github.com/WanzenBug/pylint-import-requirements/issues/17
-- if python3 -c 'import sys; sys.exit(sys.version_info < (3, 8))'; then
-  pipenv graph;
-  pipenv install --dev importlib-metadata;
-  fi
 - if [ ! -z "$PANDAS_VERSION" ]; then
     pipenv install --selective-upgrade "pandas==$PANDAS_VERSION";
   fi

Pipfile

@@ -23,8 +23,11 @@ pylint-import-requirements = "*"
 pytest-cov = ">=2"
 
 # python3.5 compatibility
-# https://github.com/python/importlib_metadata/commit/107f9029fd5807c6579b881db19e11a0488f0675
-importlib-metadata = "<3"
+# <2 for:
+# > pkg_resources.VersionConflict: (importlib-metadata 2.1.1 (/home/travis/virtualenv/python3.5.10/lib/python3.5/site-packages),
+# > Requirement.parse('importlib-metadata<2,>=0.12; python_version < "3.8"'))
+# <3 for "python_requires" https://github.com/python/importlib_metadata/commit/107f9029fd5807c6579b881db19e11a0488f0675
+importlib-metadata = "<2,<3"
 isort = "<5"
 # workaround https://github.com/pytest-dev/pytest/issues/3953
 pathlib2 = {version = "*", markers="python_version < '3.6'"}