[issue26521] add `extend_enum` to Enum

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

To me, the essential property of enums in other languages is that there is known limited set of possible values. If this set is dynamically extended, this is not enum.

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nosy: +serhiy.storchaka

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue26521>
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