Support 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.
• Fix thread-safety issues in the Cython extension module.

Are there changes in behavior for the user?

No.

Related issue number

Resolves #1455

Checklist

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

[codspeed-hq]

CodSpeed Performance Report

Merging #1456 will improve performances by 7.24%

_{Comparing lysnikolaou:free-threading-support (dc7031d) with master (8579303)}

Summary

⚡ 1 improvements
✅ 100 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_path_safe	55.7 µs	51.9 µs	+7.24%

[codecov]

Codecov Report

Attention: Patch coverage is 99.23664% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.37%. Comparing base (8579303) to head (dc7031d).
Report is 38 commits behind head on master.

Files with missing lines	Patch %	Lines
packaging/pep517_backend/_backend.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1456      +/-   ##
==========================================
+ Coverage   98.93%   99.37%   +0.44%     
==========================================
  Files          32       30       -2     
  Lines        6091     6068      -23     
  Branches      365      265     -100     
==========================================
+ Hits         6026     6030       +4     
+ Misses         62       35      -27     
  Partials        3        3              

Flag	Coverage Δ
CI-GHA	99.37% <99.23%> (+0.44%)	⬆️
MyPy	98.05% <99.10%> (-0.02%)	⬇️
OS-Linux	99.44% <100.00%> (+0.65%)	⬆️
OS-Windows	99.48% <ø> (+0.65%)	⬆️
OS-macOS	99.33% <100.00%> (+0.76%)	⬆️
Py-3.10.11	99.12% <100.00%> (+0.57%)	⬆️
Py-3.10.16	?
Py-3.10.17	99.35% <100.00%> (+0.65%)	⬆️
Py-3.11.11	?
Py-3.11.12	99.35% <100.00%> (?)
Py-3.11.9	99.12% <100.00%> (+0.57%)	⬆️
Py-3.12.10	99.35% <100.00%> (?)
Py-3.12.9	99.12% <100.00%> (+0.37%)	⬆️
Py-3.13.2	99.12% <100.00%> (+0.37%)	⬆️
Py-3.13.3	99.35% <100.00%> (?)
Py-3.13.3t	99.75% <ø> (?)
Py-3.9.13	99.07% <100.00%> (+0.57%)	⬆️
Py-3.9.21	?
Py-3.9.22	99.30% <100.00%> (+0.80%)	⬆️
Py-pypy7.3.16	99.33% <ø> (+0.64%)	⬆️
Py-pypy7.3.19	99.36% <ø> (+0.64%)	⬆️
VM-macos-latest	99.33% <100.00%> (+0.76%)	⬆️
VM-ubuntu-latest	99.44% <100.00%> (+0.65%)	⬆️
VM-windows-latest	99.48% <ø> (+0.65%)	⬆️
pytest	99.44% <100.00%> (+0.65%)	⬆️

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[bdraco]

Thanks @lysnikolaou & thanks for your patience with this one