gh-119525: Fix deadlock with `_PyType_Lookup` and the GIL (#119527)

gh-119525: Fix deadlock with `_PyType_Lookup` and the GIL (#119527)

The deadlock only affected the free-threaded build and only occurred
when the GIL was enabled at runtime. The `Py_DECREF(old_name)` call
might temporarily release the GIL while holding the type seqlock.
Another thread may spin trying to acquire the seqlock while holding the
GIL.

The deadlock occurred roughly 1 in ~1,000 runs of `pool_in_threads.py`
from `test_multiprocessing_pool_circular_import`.