require unittest.mock instead of mock

[pgajdos]

Could you please consider to use unittest.mock instead of mock, as fallback at least?

[dvzrv]

Hi! I'm packaging this project for Arch Linux and would also very much like to see this happen.
Is there a particular reason to still offer support for python 2.7? Only with python >= 3.3 unittest.mock is available and it's possible to replace mock (entirely). If backwards compatibility should still be guaranteed, it would need to be done via a conditional import.

In general it would be advisable to not support versions outside of the officially supported Python versions though, as that guarantees an easier handling of these types of dependencies (by removing them).

[dvzrv]

@vstoykov @matthewwithanm ping

[vstoykov]

Fixed in #71