Support for the free-threaded build of CPython 3.13

[lysnikolaou]

Long story short

Hey all! 👋

I took the liberty of looking into adding support for the 3.13 free-threaded build of CPython. This aims to be a tracking issue for all work necessary to support it, for which I'm going to open a PR shortly.

• [x] Set up CI for free-threading
• [x] Move away from APIs that use borrowed references (PyList_GetItem, PyDict_GetItem and friends) under the free-threaded build
• [x] Audit the C extension module for thread-safety issues
  • [x] There currently only seem to be issues with pair_list_global_version. If there's something else that I might've missed, please let me know.
• [x] Mark the C extension module as thread-safe with PyUnstable_Module_SetGIL
• [x] Build wheels for the free-threaded build.

Hope this all sounds good! Let me know in case there's something that doesn't seem right.

[ngoldbaum]

It looks like all the checkmarks are ticked, can this be closed?

[lysnikolaou]

I think it can be closed, but I'll let someone from the core team push the button.