Proper free-threaded ABI support blockers

[nitzmahone]

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

• Extensions built with Py_LIMITED_API (default) under t ABI will always segfault. (fixed by Hard disable Py_LIMITED_API under free-threaded build #125)
• Threaded embedding tests fail under free-threaded build. ([Python bug]
  (Crash in Py_Initialize in non-main thread in free-threading build python/cpython#123022), fixed in 3.13.0rc2)
• 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 Remove pre-Python 3.8 C code #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.

[colesbury]

Hi @nitzmahone - I'm happy to help out with this if you're okay with reviewing some PRs.