Fix Python string escape warning

Author: adamchainz

test_project/select2_one_to_one/urls.py

@@ -18,7 +18,7 @@
         name='select2_one_to_one_autocomplete',
     ),
     url(
-        'test/(?P<pk>\d+)/$',
+        r'test/(?P<pk>\d+)/$',
         generic.UpdateView.as_view(
             model=TModel,
             form_class=TForm,