Implement support for the free-threaded build of CPython 3.13

[lysnikolaou]

What do these changes do?

• Add a matrix config for free-threaded Python to run tests with it.
• Change configuration to correctly build wheels under the free-threaded build.

Are there changes in behavior for the user?

No.

Related issue number

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes

[webknjaz]

I think this can go into pyproject.toml. There's a section for Cython.

[lysnikolaou]

I tried that but it unfortunately cannot, because there's not the ability to add a version-specific option to pyproject.toml. For this, the build backend would have to be changed, which seems like a bigger hassle.

[lysnikolaou]

Hey folks, thanks for all the review and sorry for the extended delay here, I was out on PTO. I've addressed most of the comments around adding some TODOs and replied to the rest.

[codspeed-hq]

CodSpeed Performance Report

Merging #84 will degrade performances by 7.82%

_{Comparing lysnikolaou:freethreading-support (880daae) with master (0fc162b)}

Summary

❌ 1 regressions
✅ 3 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	master	lysnikolaou:freethreading-support	Change
❌	test_under_cached_property_cache_hit	68.1 µs	73.9 µs	-7.82%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.03%. Comparing base (0fc162b) to head (880daae).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   89.02%   89.03%   +0.01%     
==========================================
  Files          18       18              
  Lines         738      739       +1     
  Branches       75       75              
==========================================
+ Hits          657      658       +1     
  Misses         63       63              
  Partials       18       18              

Flag	Coverage Δ
CI-GHA	89.03% <100.00%> (+0.01%)	⬆️
MyPy	78.53% <100.00%> (+0.03%)	⬆️
OS-Linux	99.44% <100.00%> (ø)
OS-Windows	95.41% <ø> (ø)
OS-macOS	95.86% <100.00%> (ø)
Py-3.10.11	95.59% <100.00%> (ø)
Py-3.10.16	97.79% <100.00%> (ø)
Py-3.11.11	97.79% <100.00%> (ø)
Py-3.11.9	95.59% <100.00%> (ø)
Py-3.12.8	97.79% <100.00%> (ø)
Py-3.13.1	97.55% <ø> (-0.25%)	⬇️
Py-3.9.13	95.58% <100.00%> (ø)
Py-3.9.21	97.79% <100.00%> (ø)
Py-pypy7.3.16	97.23% <ø> (ø)
Py-pypy7.3.17	97.24% <ø> (ø)
VM-macos-latest	95.86% <100.00%> (ø)
VM-ubuntu-latest	99.44% <100.00%> (ø)
VM-windows-latest	95.41% <ø> (ø)
pytest	99.44% <100.00%> (ø)

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