migrate to unittest.mock and do not run tests on python 2

[jblanckelkoo]

Replaces #1029

[bittner]

I think, as long as Python 2.7 is supported tests should run for it. For what I can see there are two options:

• Either release the current state as "the last version supporting 2.7" and drop Python 2.7 support completely after the release (hence this or a similar PR).
• Or add try: ... except: ... constructs to the code base to make our code work for both the independent mock package and the Python library's unittest.mock.

Personally, I'd rather go for the first option. But how realistic it is that this will actually happen, you have to tell, @jenisys. It would be a long overdue thing to do, I feel, and would make a lot of people happy (aka restore their trust in this project).