[3.13] gh-117657: Fix race involving GC and heap initialization (GH-119923) (#120038)

The `_PyThreadState_Bind()` function is called before the first
`PyEval_AcquireThread()` so it's not synchronized with the stop the
world GC. We had a race where `gc_visit_heaps()` might visit a thread's
heap while it's being initialized.

Use a simple atomic int to avoid visiting heaps for threads that are not
yet fully initialized (i.e., before `tstate_mimalloc_bind()` is called).

The race was reproducible by running:
`python Lib/test/test_importlib/partial/pool_in_threads.py`.
(cherry picked from commit e69d068ad0bd6a25434ea476a647b635da4d82bb)

Co-authored-by: Sam Gross <colesbury@gmail.com>

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