[3.15] gh-153772: Make abc isinstance() tolerate instances without __class__ (GH-154149) - #156541

…lass__ (pythonGH-154149)

The built-in isinstance() reads an instance's __class__ with a lookup that
suppresses AttributeError and falls back to the object's type, so
isinstance(obj, int) returns False for an object whose __class__ access
raises. ABCMeta.__instancecheck__ read __class__ directly instead, so
isinstance(obj, Mapping) leaked that AttributeError.

Fall back to type(instance) when __class__ is unavailable, in both the C
and the pure-Python implementations, so the abstract base classes behave
like the built-in isinstance(). Such objects are unusual, but they do turn
up in the wild (for example some Qt widgets).
(cherry picked from commit 3e245faf34efc41c39bbe76071e6edb7a62692a8)

Co-authored-by: Vyron Vasileiadis <hi@fedonman.com>