Run tests with free-threaded python

[kmnhan]

Track support for free-threaded python builds.

Will probably take a long time for all upstream packages to enable support.

Now that numba supports 3.14t starting from 0.65.0, the blocker is PyQt6/PySide6.

PyQt6 will probably support it with Python 3.15, related issues are Python-SIP/sip#34 and Python-SIP/sip#35

Things are less clear for PySide6, but we could expect a similar timeline.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.31%. Comparing base (ff860be) to head (1d30b90).
⚠️ Report is 258 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #99      +/-   ##
==========================================
- Coverage   85.32%   85.31%   -0.02%     
==========================================
  Files         111      111              
  Lines       26116    26116              
  Branches     4131     4131              
==========================================
- Hits        22284    22281       -3     
- Misses       2522     2523       +1     
- Partials     1310     1312       +2     

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