Support for Python 3.13 freethreading

[jcrist]

In #711 we added support for Python 3.13. More work is needed to add support for freethreading (and build wheels supporting it).

• Add CI support
• Synchronize around caches
• Synchronize around critical sections (I believe these mostly exist in serialization)
• Ensure type parsing logic and caching is threadsafe
• Expand tests to add threading tests

[gokhanmeteerturk]

I don’t have enough C expertise to contribute directly to this, but I’m really interested to know if the support for freethreading is expected to be implemented in the future. Would be nice to start a discussion around this. Is there anyone who could share some guidance on the approach, or perhaps whether this might be planned for 3.14 instead?

[rgommers]

@kumaraditya303 has been working on this, and I believe is close to ready to open a PR with all necessary thread-safety changes.

[ofek]

The aforementioned PR is open but there will be no support for 3.13 as there are several CPython issues with it that won't be fixed.

[jacopoabramo]

@ofek just out of curiosity, what are the specific problems with support for 3.13t? Thanks in advance!

[ofek]

No idea cc @kumaraditya303

[kumaraditya303]

what are the specific problems with support for 3.13t? Thanks in advance!

3.13 has very experimental support for free-threading and the critical section implementation has weaker guarantees, specifically 3.14 implements python/cpython#114203 which makes critical section more like recursive locks.