gh-117657: Fix race involving GC and heap initialization (#119923)

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`.

Python/pystate.c

@@ -3074,6 +3074,8 @@ tstate_mimalloc_bind(PyThreadState *tstate)
     // _PyObject_GC_New() and similar functions temporarily override this to
     // use one of the GC heaps.
     mts->current_object_heap = &mts->heaps[_Py_MIMALLOC_HEAP_OBJECT];
+
+    _Py_atomic_store_int(&mts->initialized, 1);
 #endif
 }