Remove the use of mock

[dvzrv]

Hi! I'm packaging this project for Arch Linux.

In the tests, this project makes extensive use of mock, which is deprecated in favor of unittest.mock (with Python >= 3.3).

I could open a PR for removing the use of mock in favor of unittest.mock, as I assume it does not make any sense to further support anything outside the range of supported Python versions (especially nothing below Python 3.3).

[dvzrv]

I saw that you are still testing against python 2.7. Maybe that is intentional? Either way, python2 has been deprecated for a very long time and is removed from most distributions by now.

In case you still want to be backwards compatible, you may use something similar to what I offered in #150.
I've just closed the PR again, as I am not willing to sign a CLA. Sorry.