Overzealous Optimization For `isinstance`

Bug report

Bug description:

The custom __instancecheck__ dunder method allows full customization (described in the Data model).

However, it produces a spurious result:

class CustomIsInstanceMeta(type):
    def __instancecheck__(cls, instance: object) -> bool:
        return type(instance) is int

class CustomIsInstance(metaclass=CustomIsInstanceMeta):
    ...

class CustomIsInstanceSubclass(CustomIsInstance):
    ...


print(isinstance(5, CustomIsInstance)) # True
print(isinstance(CustomIsInstance(), CustomIsInstance)) # True (should be False)
print(isinstance(CustomIsInstanceSubclass(), CustomIsInstance)) # False

This is caused by:
https://github.com/python/cpython/blob/23c488d6197191d355be6733699a295c20806932/Objects/abstract.c#L2641-L2644
which is invalid in this case.

This is a reframing of #79264 (request to fix documentation)

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-144874
• gh-144997

Review Objects/abstract.c at lines 2641-2644 and the linked PRs gh-144874 and gh-144997. Run the supplied isinstance reproduction on Python 3.13/Linux to confirm the spurious results. Done means the reported behavior is resolved, with the outcome aligned with the documentation concern raised in #79264.