Drop support for python 3.6

[samj1912]

Thanks for submitting a PR!

Please make sure to check for the following items:

• Add unit tests and integration tests where applicable.
  If you've added an error code or changed an error code behavior,
  you should probably add or change a test case file under tests/test_cases/ and add
  it to the list under tests/test_definitions.py.
  If you've added or changed a command line option,
  you should probably add or change a test in tests/test_integration.py.
• Add a line to the release notes (docs/release_notes.rst) under "Current Development Version".
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Please don't get discouraged as it may take a while to get a review.

[RuRo]

Sorry, but can you explain, why was this done? PEP494 seems to suggest, that Python3.6 EOL is December 2021.

[samj1912]

Python 3.6 has not been getting bug fixes since 3.6.8 (which was released in 2018). The latest tests were failing on 3.6 on macos. I do not want to block PRs on it and I do not have the time to put in a proper fix for such an old version of Python (released 5 years ago). I have currently staged this removal for the next major release of pydocstyle. If someone wants to fix the tests and add back support for 3.6 before the next major version is published, they are more than welcome to submit a PR with the updated GH workflow tests to add back support for 3.6.

[samj1912]

Created an issue to track adding back support for 3.6 in case anyone is interested in creating a PR to fix it #540