[issue22735] Fix various crashes exposed through mro() customization

[Eldar Abusalimov]

Eldar Abusalimov added the comment:

This is a patch with most significant changes, please review it carefully.

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Added file: http://bugs.python.org/file37032/0007-fix-handle-tp_mro-overwritten-through-reentrancy.patch

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue22735>
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