gh-120321: Make gi_yieldfrom thread-safe in free-threading build

[colesbury]

Add a FRAME_SUSPENDED_YIELD_FROM_LOCKED state that acts as a brief lock, preventing other threads from transitioning the frame state while gen_getyieldfrom reads the yield-from object off the stack.

• Issue: SIGSEGV with generators in free-threaded build #120321

[mpage]

LGTM. I worry a little about the increasing complexity in the state machine. Specifically, that folks might forget to call gen_yield_from_lock_wait() at the beginning of the state transition loop. Would it be worth factoring out the common logic?

[colesbury]

Hmmm.. maybe we can handle it in _Py_GEN_TRY_SET_FRAME_STATE

[colesbury]

I've move the retry logic into _Py_GEN_TRY_SET_FRAME_STATE. @mpage would you please take a look at the most recent changes?

[mpage]

Nice!