[issue27877] Add recipe for "valueless" Enums to docs

[John Hagen]

John Hagen added the comment:

Raymond, thanks for your consideration and input.

I'll work on a small patch unless I hear from Ethan that he'd rather do it.  I'm happy to defer to his expertise.

I did try out None as a value just to be sure that that didn't work as it would not be a bad alternative.  But since the values are "equal", Enum tries to alias them.

import enum

@enum.unique
class Color(enum.Enum):
    red = None
    green = None
    blue = None

>>> ValueError: duplicate values found in <enum 'Color'>: green -> red, blue -> red

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<https://bugs.python.org/issue27877>
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