gh-111926: Avoid locking in PyType_IsSubtype (#117275)

Read the MRO in a thread-unsafe way in `PyType_IsSubtype` to avoid locking. Fixing this is tracked in #117306. 

The motivation for this change is in support of making weakrefs thread-safe in free-threaded builds:

`WeakValueDictionary` uses a special dictionary function, `_PyDict_DelItemIf`
to remove dead weakrefs from the dictionary. `_PyDict_DelItemIf` removes a key
if a user supplied predicate evaluates to true for the value associated with
the key. Crucially for the `WeakValueDictionary` use case, the predicate
evaluation + deletion sequence is atomic, provided that the predicate doesn’t
suspend. The predicate used by `WeakValueDictionary` includes a subtype check,
which we must ensure doesn't suspend in free-threaded builds.