Investigate fixing CI for python 3.6

[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.

Originally posted by @samj1912 in #539 (comment)

Python 3.6 support was removed in the above PR. Creating this issue to track if adding back support for 3.6 is feasible before the next major release of pydocstyle.