Fix crash with PartialTypes and the enum plugin

[Michael0x2a]

Fixes #12109.

The original issue reported that the bug had to do with the use of the --follow-imports=skip flag. However, it turned out this was a red herring after closer inspection: I was able to trigger a more minimal repro both with and without this flag:

from enum import Enum

class Foo(Enum):
    a = []  # E: Need type annotation for "a" (hint: "a: List[<type>] = ...")
    b = None

    def check(self) -> None:
        reveal_type(Foo.a.value)  # N: Revealed type is "<partial list[?]>"
        reveal_type(Foo.b.value)  # N: Revealed type is "<partial None>"

The first two reveal_types demonstrate the crux of the bug: the enum plugin does not correctly handle and convert partial types into regular types when inferring the type of the .value field.

This can then cause any number of downstream problems. For example, suppose we modify def check(...) so it runs reveal_type(self.value). Doing this will trigger a crash in mypy because it makes the enum plugin eventually try running is_equivalent(...) on the two partial types. But is_equivalent does not support partial types, so we crash.

I opted to solve this problem by:

1. Making the enum plugin explicitly call the fixup_partial_types function on all field types. This prevents the code from crashing.

2. Modifies mypy so that Final vars are never marked as being PartialTypes. Without this, reveal_type(Foo.b.value) would report a type of Union[Any, None] instead of just None. (Note that all enum fields are implicitly final).

(Explain how this PR changes mypy.)

[github-actions]

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

[JukkaL]

One more crash fixed, nice! Looks good.