use unittest.mock instead of mock

[pgajdos]

I see you are still supporting Python 2, so dropping mock usage is not really an option, but could you please consider to fallback to unittest.mock when mock is not available?

Background:
https://trello.com/c/S6eADbii/64-remove-python-mock
https://fedoraproject.org/wiki/Changes/DeprecatePythonMock