[requirements][all] migration OFF of mock to the unittest.mock built-in library

I'd like to suggest, now that we are on modern python, that we stop using the mock library and instead use the built in mock library. unittest.mock has been around since python-3.3 so we should not have a problem with python support. This would allow us to drop a library and also help prevent issues with stuff like https://review.opendev.org/712713 is going to expose (nova/neutron fail tests with the external mock-4 library). -- Matthew Thode