chore(package): add Python 3.13 classifier

[mkniewallner]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• If you've added or modified a feature, documentation in docs is updated

Description of changes

With #713 merged, we now test against Python 3.13. Since tests work as expected, we can add Python 3.13 classifier to hint at it being supported.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.0%. Comparing base (6d2344b) to head (0bf96d8).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #809   +/-   ##
=====================================
  Coverage   93.0%   93.0%           
=====================================
  Files         37      37           
  Lines        971     971           
  Branches     177     177           
=====================================
  Hits         904     904           
  Misses        51      51           
  Partials      16      16           

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