Use compatability layer for user in test

tests/unit/wishlist_tests.py

 from django.test import TestCase
-from django.contrib.auth.models import User
 
 from oscar.apps.wishlists.models import WishList
+from oscar.core.compat import get_user_model
+
+User = get_user_model()
 
 
 class TestAWishlist(TestCase):