Python 3.11 was released 2023-10-02.

Python dropped support on 2021-12-23. Our policy (#760) is to drop
support on the next point release after Python does.

* Remove py36 tests
* Test multiple recent versions of pypy
* Remove pep562 backport

Replaces CLI action run through tox.
Closes #1243.

Fixes #1124.

If cache is desired, we should look into this in the future and do it
properly.

This action checks that an update was made to any file in docs/change_log/
but only if changes were made to files in markdown/. Presumably,
any changes outside of markdown/ do not affect the behavior and do not
require a notation in the change_log.

If the proper permissions are available, a comment is added to the PR informing
the PR author that an update to the changelog is missing. However, any PR's from
forks do not have permission on the pull_request event. Unfortunately, the
pull_request_target event doesn't seem to work at all. However, as the action
only attempts to add a comment on failure, we only get a failure in the correct
conditions, even if the error message is related to auth.

Python 3.5 reached end-of-life on 2020-09-12 and Python 3.9 was released on 2020-10-05.