[issue33018] Improve issubclass() error checking and message

[INADA Naoki]

INADA Naoki <songofacandy at gmail.com> added the comment:

If there are some code which depend on ABC.__subclasscheck__() allow class-like object, we'll have to revert it in 3.7b4 or rc.
I think it's the only risk.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33018>
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