Add support for pytest 9 and drop Python 3.9 support

[sanders41]

Pytest 9 dropped support for Python 3.9 so that is why I dropped it here.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.46%. Comparing base (03dd96a) to head (913d262).
⚠️ Report is 74 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1278      +/-   ##
==========================================
- Coverage   90.53%   90.46%   -0.07%     
==========================================
  Files           2        2              
  Lines         412      409       -3     
  Branches       45       44       -1     
==========================================
- Hits          373      370       -3     
  Misses         30       30              
  Partials        9        9              

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[sanders41]

I assume the timeline protection is wanting me to add something for the changelog? I can't find any documentation on what it is looking for.

[Pierre-Sassoulas]

The message from chronographer is easily missable : No files matching re.compile('changelog.d/(?P<issue_number>[^\\./]+).(?P<fragment_type>security|removed|deprecated|added|changed|fixed|downstream)(.\d+)?(.[^\\./]+)*$') pattern added

[Pierre-Sassoulas]

LGTM, but I'm not in a position to merge this.

[seifertm]

Thanks @sanders41 !

I split up the existing news fragment, so that the removal of Python 3.9 support and the support for pytest 9 ends up in the Removed and Added sections of the changelog, respectively. Then I squashed the commits to clean up the history.

Big thanks to @Pierre-Sassoulas as well for helping out! Honestly, I wouldn't mind you merging this PR.

Release will follow in a couple of hours. I just want to check if it makes sense to include some of the other open PRs in the release.