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

[Eldar Abusalimov]

Changes by Eldar Abusalimov <eldar.abusalimov at gmail.com>:


Added file: http://bugs.python.org/file37037/0012-minor-mro_implementation-pmerge-refactory.patch

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