Py3: use unittest.mock instead of mock (#3481)

* Py3: use unittest.mock instead of mock

* noqa

* oops

* just pull in patch()

* ignore RuntimeError when stopping patch

* ignore RuntimeError from default_session_mock.stop()

tests/test_cloudformation/test_stack_parsing.py

@@ -2,8 +2,8 @@ from __future__ import unicode_literals
 import json
 import yaml
 
-from mock import patch
 import sure  # noqa
+from tests.compat import patch
 
 from moto.cloudformation.exceptions import ValidationError
 from moto.cloudformation.models import FakeStack