gh-153772: Make abc isinstance() tolerate instances without __class__

[fedonman]

The built-in isinstance() reads an instance's __class__ with a lookup that suppresses AttributeError and falls back to the object's type, so a value whose __class__ access raises still checks cleanly:

>>> class NoClass:
...     def __getattribute__(self, name):
...         if name == "__class__":
...             raise AttributeError(name)
...         return super().__getattribute__(name)
...
>>> isinstance(NoClass(), int)
False

ABCMeta.__instancecheck__ read __class__ directly instead, so the same object leaked the AttributeError when checked against a collections.abc class:

>>> from collections.abc import Mapping
>>> isinstance(NoClass(), Mapping)
AttributeError: __class__

This makes the abstract base class machinery fall back to type(instance) when __class__ is unavailable, in both the C (Modules/_abc.c) and the pure-Python (Lib/_py_abc.py) implementations, so it matches the built-in isinstance():

>>> isinstance(NoClass(), Mapping)
False

Objects without a __class__ are unusual, but they do turn up in the wild (for example some Qt widgets, as noted in the issue).

• Issue: isinstance check against collection.abc classes fails if value has no __class__ #153772

[python-cla-bot]

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