Type cache can expose previous values during finalization of an object

Bug report

Bug description:

This should always print True, that is an attribute accessed from a class should match the value that's in the type's mappingproxy:

class X:
    def __del__(inner_self):
        v = C.a
        print(v == C.__dict__['a'])

class C:
    a = X()

# prime the cache
C.a
C.a

# destructor shouldn't be able to see inconsisent state
C.a = X()
C.a = X()

But this prints False because the destructor can be run when the assignment into the type dict occurs, but before the type version is invalidated.

CPython versions tested on:

3.8, CPython main branch

Operating systems tested on:

Linux

Start by running the reproducer on CPython main and compare the attribute lookup with the type mapping during the destructor triggered by assignment. Trace the type-cache invalidation and finalization sequence; done means the example consistently prints True without exposing the inconsistent state.