Fix mistaken type annotation

[mgax]

In #179 I added a broken type annotation:

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

Since for_user is a classmethod, its first argument is a class, not an instance. The correct annotation would be:

    @classmethod
    def for_user(cls: Type[T], user: AbstractBaseUser) -> T:

I have checked both versions by running uv tool run --with=. ty check /tmp/typing-sandbox.py in the django-ninja-jwt worktree against this sample file:

# /tmp/typing-sandbox.py
from ninja_jwt.tokens import Token, RefreshToken
from django.contrib.auth.models import User

Token().for_user(User())
RefreshToken().for_user(User())

Apologies for the mistaken PR – I had tested the change locally but it seems I was not careful 🙈

I would be happy to add ty check to CI if that contribution would be accepted.