Proper free-threaded ABI support blockers

Tracking known blockers to actual support of Python's new t free-threaded ABI - add new ones as we discover them.

[x] Extensions built with Py_LIMITED_API (default) under t ABI will always segfault. (fixed by #125)
[x] Threaded embedding tests fail under free-threaded build. ([Python bug] (https://github.com/python/cpython/issues/123022), fixed in 3.13.0rc2)
[x] Add non-artifact CI job for 3.13t.
[ ] Evaluate all type creation/caching code for obvious GIL-incidental locking or deadlock issues sans GIL.
[ ] Evaluate all manual refcounting, buffer manipulations, marshaling code, etc for obvious concurrency issues.
[ ] Build a lot more threaded stress tests for the above, measure coverage.
[ ] Update documentation to describe current state, concerns around GIL-free operation modes.
[ ] Once we have some confidence in true GIL-free operations, provide a high-level opt-in/out for CFFI-backed extensions to call PyUnstable_Module_SetGIL() during extension init to control the automatic legacy extension GIL re-enable.
[ ] Remove known dead code from active development branches (ala #124).
[ ] Work with high-profile CFFI-consuming projects (e.g. cryptography, pynacl, pygame, ?) to ensure any future threaded stress tests are not broken by CFFI issues.

python-cffi / cffi