Add six.moves.unittest_mock

[ddmee]

Unittests mock module was added in python 3, it's called unittest.mock in python 3 but it was made available as a backport for python 2 via pypi as mock.

The information is listed on the python 3 documentation https://docs.python.org/3/library/unittest.mock.html

unittest.mock — mock object library
New in version 3.3.
Source code: Lib/unittest/mock.py
...
There is a backport of unittest.mock for earlier versions of Python, available as mock on PyPI.

Mock on pypi: https://pypi.org/project/mock/

This lets a user write six.moves.unittest_mock which is handy for projects like mine that use mock on python 2 and 3.

Hope this PR is alright. I also updated the changes file and version number etc, as it seemed the right thing to do.

[ddmee]

I don't think that CI failure is anything to do with this PR