Refactor 'NewType' as it no longer conforms to the typing spec

[mvanderlee]

Refactor 'NewType' as it no longer conforms to the typing spec

[dairiki]

Not a problem, but something to be aware of:

The proposed use of Annotated types is not equivalent to the old custom NewType with regards to static type checking. (NewType introduces a new type, while Annotated adds annotations to an existing type.)

Consider:

from typing import Annotated
from marshmallow import fields
from marshmallow_dataclass import dataclass, NewType

Email1 = NewType("Email", str, field=fields.Email)
Email2 = Annotated[str, fields.Email]

@dataclass
class Record:
    email1: Email1
    email2: Email2

r = Record(email1="[email protected]", email2="[email protected]")

Mypy will complain that Argument "email1" to "Record" has incompatible type "str"; expected "Email1". Passing the str for email2 is fine.

[dairiki]

Python is updating the Typing spec which causes Variable not allowed in type expression for types created with marshmallow_dataclass's NewType

FWIW: For mypy, at least, use of the marshmallow_dataclass.mypy mypy plugin fixes this.