bpo-44356: [Enum] allow multiple data-type mixins if they are all the same (GH-26649) (GH-26653)

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):
        GH- some behavior here

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

    class FinalStrEnum(Str1Enum, Str2Enum):
        GH- this now works
(cherry picked from commit 8a4f0850d75747af8c96ca0e7eef1f5c1abfba25)

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>