Support for free-threaded CPython

I'm filing this issue as a placeholder for potential work around supporting free-threaded (v3.13t +) Python.

The standard TODOs for adding free-threading support are:

• Audit Python bindings and declare them free-threading compatible (xref https://py-free-threading.github.io/porting/#updating-extension-modules).
• Run the test suite with pytest-run-parallel to find potential issues, and fix them.
• Run the test suite under ThreadSanitizer. If possible, depends on how many dependencies there are and if they run under TSan.
• Add cp313t-* to CI to build free-threading wheels.

For more details, please see the suggested plan of attack in the py-free-threading guide.

Note that this is the first time I've looked at this repo, so I might be missing known issues or code that needs closer inspection. Any suggestions here will be very useful.