`admin_user` fixtures assume that the user has an email field

[orf]

If you have a user model that doesn't have an email field then the admin_user fixture does not work.

In the fixture it blindly passes 'admin@example.com' as the email, but the create_superuser function might not take an email parameter.

E.G:

    def _create_user(self, username, password, **extra_fields):
        pass

    def create_user(self, username, password=None, **extra_fields):
        pass

    def create_superuser(self, username, password, **extra_fields):
        pass

This works for manage.py createsuperuser but not for admin_user