Enums: Teach mypy that descriptors are not converted to become enum members

[AlexWaygood]

Fixes #12494.

Description

At runtime, most objects in an enum class statement are automatically converted by the enum module into members of the enumeration. However, if an object has a __get__, __set__ or __delete__ method, special-casing by the enum module means that the object is not converted into an enum member. Mypy is currently aware of this special-casing, leading it to falsely assume that certain enum classes are unsubclassable, when, in fact, they are.

This PR teaches mypy about descriptor special-casing in enums.

Test Plan

• Two test cases have been added.
• One test case has been extended.
• One test case has been altered slightly -- it was in fact buggy. (It shouldn't have been passing.)

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[AlexWaygood]

I doubt I'll get to this any time soon

[finite-state-machine]

For what it's worth: this issue is very difficult to work around using type: ignore or cast(), especially if the Enum subclass is in a library: you'd have to type: ignore every place where the descriptor was referenced.

Any suggestions would be most welcome.