feat: if for free-threading

[henryiii]

I think this won't work on Windows yet, we'll need to find the flags another way there, I think. Now uses packaging.tags. Close #770.

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.70%. Comparing base (6afb1d6) to head (ac07c66).
Report is 62 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/scikit_build_core/settings/skbuild_overrides.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #819      +/-   ##
==========================================
+ Coverage   83.68%   83.70%   +0.01%     
==========================================
  Files          74       74              
  Lines        4204     4227      +23     
==========================================
+ Hits         3518     3538      +20     
- Misses        686      689       +3     

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