🩹 make sure to define Py_GIL_DISABLED on Windows for Python 3.13t

[burgholzer]

Description

As pointed out in pypa/cibuildwheel#1975 (comment) free-threading (Python 3.13t) builds on Windows need to have Py_GIL_DISABLED set in order to properly work.
This PR ensures that the corresponding setting is always set.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.4%. Comparing base (752cec2) to head (f9457b7).
Report is 190 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #513     +/-   ##
=======================================
- Coverage   90.4%   90.4%   -0.1%     
=======================================
  Files         86      86             
  Lines      10024   10025      +1     
  Branches    1711    1711             
=======================================
- Hits        9070    9067      -3     
- Misses       954     958      +4     

Flag	Coverage Δ
cpp	90.1% <ø> (-0.1%)	⬇️
python	96.0% <ø> (ø)

see 2 files with indirect coverage changes