Fix test_scalar_list.py for python3

tests/types/test_scalar_list.py

@@ -53,7 +53,7 @@ class TestScalarUnicodeList(TestCase):
             "List values can't contain string ',' (its being used as "
             "separator. If you wish for scalar list values to contain "
             "these strings, use a different separator string.)"
-        ) in db_err.value.message
+        ) in str(db_err.value)
 
     def test_save_unicode_list(self):
         user = self.User(