EnumMetaclass should have a __instancecheck__ method

[Barry Warsaw]

There's been some discussion on python-dev that enum items pass an instance check against their enum class. E.g.

>>> X = Enum('X', 'a b c')
>>> Y = Enum('Y', 'z y x')
>>> class Z(Y):
... d = 4
... e = 5
...
>>> isinstance(Z.d, Y)
True
>>> isinstance(Z.d, Z)
True
>>> isinstance(Z.d, X)
False
>>> isinstance(Y.z, Y)
True
>>> isinstance(Y.z, Z)
False

To do this, we could add the following method to EnumMetaclass:

    def __instancecheck__(cls, instance):
        return instance.enum is cls or cls in instance.enum.__bases__

I haven't decided whether it's a good idea or not though, since the semantics seem weird to me.