Support enum member `value` types

Not sure if this would be in mypy or the typeshed, though it may need support in both.

Currently enum.IntEnum provides a way to have an enum whose value is an int, however all other enum.Enums have a .value of Any.

While this can be worked around in more recent versions of Python by adding a member without a value within the enum:

class MyEnum(str, enum.Enum):
    value: str
    FIRST = 'first'

This isn't ideal as it means that there are two places where this needs to be specified -- as an inherited type and within the class body.

It feels like it would be better if mypy was able to infer the member value type from the inheritance.

Aside: had generics existed before Enum, it feels like that might alternatively solved this, something like:

class MyEnum(enum.Enum[str]):
    FIRST = 'first'

Edit:
Note that this issue is about the general use-case of variables typed for the enum (such as x: MyEnum) -- mypy has long supported determining the type of specific members and their .values (e.g: reveal_type(MyEnum.FIRST.value) is already str or more recently Literal['first']).

I've put a more complete demo of this at https://gist.github.com/PeterJCLaw/d4334e498ab1391ee306886748e675f1.

まず issue の完全なデモを確認し、mypy と typeshed における enum の処理を追跡します。issue では特定のファイルやテストは示されていません。enum 変数の値の型を継承からどのように推論すべきかを判断し、その後、一般的な enum インスタンスがクラス本体の個別のアノテーションなしで意図した値の型を公開することを示すカバレッジを追加します。