[Typing-sig] generics support for Enum

Greetings! I'd like to add Generics support for Enum... at least I think I do. :-/ The magic seems to happen in `SomeMetaClass.__getitem__`, and it should be doable as `EnumType.__getitem__` currently only accepts strings, so types could easily take a different branch in the code, but I am entirely unsure of what that separate branch should do, what it should return, etc. Are there any pure-python classes in the stdlib (besides Generics) that I could use for inspiration? Or some documentation somewhere I could use? A basic tutorial so I can get a grip on all the terminology? Any and all help appreciated! -- ~Ethan~