[issue33018] Improve issubclass() error checking and message

[INADA Naoki]

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

My current opinion is:

* -1 for 3.6:  Behavior should not be changed without strong reason, even the behavior is not documented.
* +1 for 3.8:  I like strict and less magic.
* +0 for 3.7:  beta3 is bit late, but this change has very little chance to cause real world problem.

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