[issue29577] Enum: mixin classes don't mix well with already mixed Enums

[Ethan Furman]

Change by Ethan Furman <ethan at stoneleaf.us>:


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versions: +Python 3.8 -Python 3.6, Python 3.7

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue29577>
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