Fix return type of the for_user method

[mgax]

The for_user classmethod returns an instance of cls(), but it claims to return Token:

django-ninja-jwt/ninja_jwt/tokens.py

Lines 183 to 184 in b419409

	@classmethod
	def for_user(cls, user: AbstractBaseUser) -> "Token":

To get the access token from RefreshToken.for_user, one has to cast the return value in order to make the type checker happy:

refresh_token = cast(RefreshToken, RefreshToken.for_user(user))
access_token = refresh_token.access_token

This PR removes the need for that cast by annotating for_user to return Self, which may be Token, or the subclass.

[mgax]

I tried to use typing.Self at first but it was only introduced in Python 3.11 and the testsuite checks against older Pythons.