Fix CI

[hynek]

Hey, I just made a Pull Request!

Description

Drops unsupported Python versions, adds supported ones and make the CI green again.

Code changes except for the pkg_resources fix are all mechanical by pyupgrade.

Checklist for PR author(s)

• [x] Changes are covered by unit tests (no major decrease in code coverage %).
• [x] All tests pass.
• [x] Docstring coverage is 100% via tox -e docs or interrogate -c pyproject.toml (I mean, we should set a good example :smile:).
• [x] Updates to documentation:
  • [x] Document any relevant additions/changes in README.rst.
  • [x] Manually update both the README.rst and docs/index.rst for any new/changed CLI flags.
  • [x] Any changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives. Find the appropriate next version in the project's __init__.py file.

Release note

Added
^^^^^

- Support for Python 3.11 & 3.12.

Removed
^^^^^^^

- Support for Python 3.6 & 3.7.

[hynek]

After this is merged, please:

• [x] go to https://pre-commit.ci and sign up for it. It's much faster than anything and will automatically keep your .pre-commit-config.yaml updated.
• [x] enable RTD PR builds https://docs.readthedocs.io/en/stable/pull-requests.html
• [x] add them plus required-checks-pass to Settings/Branches/master/"Require status checks to pass before merging".