[issue33018] Improve issubclass() error checking and message

[Ivan Levkivskyi]

Ivan Levkivskyi <levkivskyi at gmail.com> added the comment:

> Would you merge this into master?

OK, I played with this a bit and it looks good. There is however a merge conflict now, and a NEWS item is missing. I will leave a comment in the PR.

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<https://bugs.python.org/issue33018>
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