Implement support for the free-threaded build of CPython 3.13

[lysnikolaou]

What do these changes do?

Implement support for the free-threaded build of CPython 3.13!

In more detail, this PR does the following:

• Add a CI job that runs the tests under the free-threaded build.
• Enable building free-threaded wheels.
• Add pythoncapi_compat.h and move away from all the borrowed reference APIs.
• Mark extension module as thread-safe
• Add locking around pair_list_global_version.

Are there changes in behavior for the user?

There shouldn't be any.

Related issue number

Related to #1014.

Checklist

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

[bdraco]

The changes look good to me and I didn't see any ref count issues, however I am by no means an expert on the C API

[bdraco]

Assuming the CI passes (might take a while to run since there is a lot of activity right now), I'd like another maintainer who has more experience with the C API to take a look before merging.

[bdraco]

Thanks @lysnikolaou

[bdraco]

It looks like one of the existing actions unrelated to this PR is using an old upload-artifact. It would be nice if github told us which one.

[lysnikolaou]

Yeah, I saw that as well. Opened #1016 for it.

[bdraco]

Looks like its almost there 👍

[lysnikolaou]

Looks like it's good to go on my side!

[bdraco]

LGTM, but I'd still like another set of 👀 #1015 (comment) before merging

[lysnikolaou]

Friendly ping here!

[webknjaz]

Here's my CI/CD feedback. I can't say I'm experienced with C-API, however. Still, I think we can merge this and start a release process to see if more rare architectures also succeed building the new wheels. It should be enough to see if cibuildwheel passes in those jobs that only run on release, since it also runs some testing. We can then abort the release once we know. Or we could just attempt a regular release.

[lysnikolaou]

Thanks for the reviews everyone! I've addressed all of the feedback points.

[bdraco]

It looks like we don't have any benchmarks for this repository so can't check the performance impact. I guess I'll need to write some.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.86%. Comparing base (217d16e) to head (a60248d).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1015   +/-   ##
=======================================
  Coverage   94.86%   94.86%           
=======================================
  Files          23       23           
  Lines        2686     2686           
  Branches      254      254           
=======================================
  Hits         2548     2548           
  Misses         67       67           
  Partials       71       71           

Flag	Coverage Δ
CI-GHA	94.86% <ø> (ø)
MyPy	67.51% <ø> (ø)
pytest	98.11% <ø> (ø)

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[bdraco]

It looks like the testing fails because there is no cffi wheel for free-threaded 3.13 so it tries to build it and since libffi isn't installed in the cibuildwheel env, it fails

[codspeed-hq]

CodSpeed Performance Report

Merging #1015 will not alter performance

_{Comparing lysnikolaou:free-threading-support (a60248d) with master (217d16e)}

Summary

✅ 38 untouched benchmarks