Performing a logical negation (`not`) on `NotImplemented` should return `NotImplemented`. Currently, it returns `False`.

A common pattern for implementing __eq__ and __ne__ is to implement the comparison in __eq__, and simply delegate to it in __ne__ with a negation. However, if two values are incomparable, then __eq__ and __ne__ should both return NotImplemented. If you try to negate NotImplemented in __ne__, you will end up with a value of False, instead of NotImplemented, so you have to specifically test for this case.

For instance, here is how one would write the code now:

    def __ne__(self, other):
        eq = self.__eq__(other)
        if eq is NotImplemented:
            return NotImplemented
        return not eq

Where as the following would be simpler, and could be used if this change was made:

    def __ne__(self, other):
        return not self.__eq__(other)

This is not simply sugar to reduce typing, it is safer because some coders may forget about NotImplemented and implement __ne__ as shown in the second example anyway, which is not actually correct with the current behavior.