remove dependency on old mock

[a-detiste]

https://github.com/testing-cabal/mock

mock is now part of the Python standard library, available as unittest.mock in Python 3.3 onwards.

[coveralls]

coverage: 99.474%. remained the same
when pulling ca73fba on a-detiste:master
into 1173f28 on dbader:master.

[SijmenHuizenga]

Thanks!