[issue35712] Make NotImplemented unusable in boolean context

[Alex Waygood]

Alex Waygood <Alex.Waygood at Gmail.com> added the comment:

The following code now leads to a `DeprecationWarning`, but I am unclear why it should.

```
>>> from enum import Enum
>>> 
>>> class CardColour(Enum):
...     """Enumeration of the two colours in a pack of cards."""
... 
... 	BLACK = 'black'
... 	RED = 'red'
... 
... 	def other_colour(self):
...         """Given one colour, get the other one."""
... 	    return next(filter(self.__ne__, CardColour))
... 	
>>> CardColour.BLACK.other_colour()
<input>:5: DeprecationWarning: NotImplemented should not be used in a boolean context
<CardColour.RED: 'red'>
```

If I change the last line of `other_colour` to either `return next(colour for colour in CardColour if colour != self)` or `return next(colour for colour in CardColour if colour is not self)`, the warning goes away.

Is this intended behaviour?

----------
nosy: +AlexWaygood

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35712>
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