At least kill the last remaining use of mock.Mock

jsonschema/tests/test_validators.py

@@ -901,7 +901,7 @@ def test_valid_instances_are_valid(self):
             )
 
     def test_invalid_instances_are_not_valid(self):
-        errors = iter([mock.Mock()])
+        errors = iter([ValidationError("An error!")])
 
         with mock.patch.object(
             self.validator, "iter_errors", return_value=errors,