fix: UserAdmin accepts AbstractUser

[GabDug]

I have made things!

Fix an issue I introduced with UserAdmin: if you inherit from it with a custom User it would not work, only with Django base User.

If needed, can add a test case akin of:

@admin.register(CustomUser)
class CustomUserAdmin(UserAdmin[CustomUser]):
   ...

and

@admin.register(CustomUser)
class CustomUserAdmin(UserAdmin):
   ...

Any feedback is appreciated!

Related issues

[jmfederico]

Suggested change

	class UserAdmin(admin.ModelAdmin[_M], Generic[_M]):
	class UserAdmin(admin.ModelAdmin[_M]):

I do not think the Generic is needed.

[jmfederico]

Suggested change

	_M = TypeVar("_M", bound=AbstractUser, covariant=False)
	_M = TypeVar("_M", bound=AbstractUser)

covariant is False by default I think.

[jmfederico]

Suggested change

	from typing import Any, Generic, TypeVar
	from typing import Any, TypeVar

As per below comments, I do not think this is needed.

[vadimshvetsov]

Hey @GabDug, thanks for your PR. Do you plan to address comments or I can do it for you and open new PR? I need this also.

[kolanos]

Shouldn't this be checking for inheritance from AbstractBaseUser and not AbstractUser? A lot of people create a custom user model based on AbstractBaseUser, such as not including a username field.