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.

Inizia esaminando la demo completa dell’issue e seguendo la gestione degli enum in mypy e typeshed; l’issue non identifica file o test specifici. Determina come dovrebbe essere inferito il tipo del valore di una variabile enum dall’ereditarietà, quindi aggiungi una copertura che dimostri che le istanze generiche di enum espongono il tipo del valore previsto senza un’annotazione separata nel corpo della classe.