🩹 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 92.3%. Comparing base (15a0c7c) to head (c6c97fd).
Report is 64 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #425   +/-   ##
=====================================
  Coverage   92.3%   92.3%           
=====================================
  Files         37      37           
  Lines       2721    2721           
  Branches     305     304    -1     
=====================================
  Hits        2513    2513           
  Misses       208     208           

Flag	Coverage Δ
cpp	94.1% <ø> (ø)
python	87.4% <ø> (ø)

see 2 files with indirect coverage changes