Fix thread-safety for `PyType_IsSubtype` in free-threaded builds

[mpage]

Feature or enhancement

Proposal:

In #117275 we started performing reads of the MRO in PyType_IsSubtype in a thread-unsafe way. We need to read the MRO in a way that ideally doesn't lock, or at the very least locks without the possibility of suspending.

One potential solution is to use deferred reference counting to avoid locking.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response