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_packages/test_httpretty.py

@@ -1,7 +1,6 @@
 # #!/usr/bin/env python
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
-import mock
 
 from moto.packages.httpretty.core import (
     HTTPrettyRequest,