Added free-threaded CI jobs

[astrofrog]

Needs to be rebased once #600 is merged

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.49%. Comparing base (dfebf39) to head (d7d6e9a).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #601   +/-   ##
=======================================
  Coverage   87.49%   87.49%           
=======================================
  Files          49       49           
  Lines        1935     1935           
=======================================
  Hits         1693     1693           
  Misses        242      242           

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[Cadair]

We should add a job using https://pypi.org/project/pytest-run-parallel/ using on both t and not t as it can find a bunch of thread races even on gil builds

[astrofrog]

Sounds good - btw the existing failure here should be fixed by astropy/astropy#19819