Add __prepare__() to allow Python >= 3.3 to check for unique attribute names

[Barry Warsaw]

E.g. this should be an error in >= Python 3.3:

class X(Enum):
    a = 1
    b = 2
    a = 3

and also:

X = Enum('X', [('a', 1), ('b', 2), ('a', 3)])

There's no good way to do this in earlier Pythons though.

[Barry Warsaw (barry)]

After further thought and with Eli's feedback: the class syntax shouldn't raise an exception. This is just the way Python works, so we should leave that alone. The convenience API *could* do an extra check, but it doesn't seem worth being different here, so for now, I won't change this.