Generate a TypedDict or dataclass from Schema

It would be nice to be able to use Schemata with mypy or similar, e.g.

class User(Schema):
    name = fields.String()
    age = fields.Integer()


def have_a_birthday(user: User.type) -> User.type:
    user['age'] += '1'  # will fail type check
    return user  # typechecks OK

I started an inline implementation with Schema._declared_fields, but since there's nothing like a Field.deser_type it was going to be quite a hacky map by cases - isinstance(cls, fields.String) etc. - and then I realised I had nested fields to contend with and jumped out of the rabbit hole before I got too deep.

I do think this would be very nice to have though, or perhaps even go further and have a @deser_to_dataclass decorator (that's the route I was going) that re-writes the Schema so that the type is User (or whatever) itself, and User.name is annotated automatically as : str.