gh-111926: Update _PyWeakref_IS_DEAD to be thread-safe

[corona10]

see: #112250 (comment)

• Issue: Make weakref thread-safe without the GIL #111926

[colesbury]

I think this is good for now, but we'll need to come back to this again after other parts of weakrefobject.c are updated.

Py_REFCNT(obj) == 0 won't be sufficient here without the GIL. Outside of destructors, Py_REFCNT() is basically an estimate subject to race conditions. We'll want a check that ob_ref_shared == _Py_REF_MERGED, but we can't rely on that test either until we make some other changes to weakrefobject.c