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