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