bpo-44356: [Enum] allow multiple data-type mixins if they are all the same (GH-26649)

This enables, for example, two base Enums to both inherit from `str`, and then both be mixed into the same final Enum:

    class Str1Enum(str, Enum):
        # some behavior here

    class Str2Enum(str, Enum):
        # some more behavior here

    class FinalStrEnum(Str1Enum, Str2Enum):
        # this now works