As of Python 3.3, the 3rd-party `mock` package has been subsumed into the standard `unittest.mock` package. Refactoring tests to use the latter will allow us to drop `mock` as a dependency, which is currently coming in transitively through requirements/edx/paver.in. We don't actually drop the `mock` dependency in this PR. That will happen naturally in: * https://github.com/openedx/edx-platform/pull/34830