Support for free-threading CPython

[rgommers]

This aims to be a tracking issue for the changes necessary to support the free-threaded build of Python 3.13. Here is what that typically takes:

• Add a regular test job that runs the free-threaded interpreter in CI. Done in ci: add 3.13t to matrix #94
• Audit Python bindings and declare them free-threading compatible (xref https://py-free-threading.github.io/porting-extensions/). Done in Declare extension module to be free-threading compatible #99
• Run the test suite with pytest-run-parallel to find potential issues, and fix them. Done locally as part of the work on Declare extension module to be free-threading compatible #99, adding a CI job may be useful.
• Run the test suite under ThreadSanitizer (locally, not in CI). Done as part of the work on Declare extension module to be free-threading compatible #99
• Update the CI job that builds wheels to include free-threading wheels (cp313t ABI tag). Done in ci: enable free-threaded (cp313t) wheel builds #102

[rgommers]

I'll close this as completed, since all PRs were merged. Once the next release comes out (I assume in the near future, because Python 3.14 support), wheels will become available on PyPI.

Run the test suite with pytest-run-parallel to find potential issues, and fix them. _Done locally as part of the work on _

Given that this didn't find anything when running locally this is likely more maintenance overhead than can be justified for the gain, so I recommend to forget about that.