Fix deprecated admin site URL in tests

The old format was deprecated in Django 1.9, and removed in Django 2.0.

Ref: https://docs.djangoproject.com/en/stable/releases/1.9/#passing-a-3-tuple-or-an-app-name-to-include

polymorphic/tests/admintestcase.py

@@ -49,7 +49,7 @@ class AdminTestCase(TestCase):
         # Make sure the URLs are reachable by reverse()
         clear_url_caches()
         set_urlconf(tuple([
-            url('^tmp-admin/', include(self.admin_site.urls))
+            url('^tmp-admin/', self.admin_site.urls)
         ]))
 
     def get_admin_instance(self, model):