testing/py3-lib_users: upgrade to 0.15
Fix some Python 3 warnings
Hanno reported the unclosed FD warnings caused by me not using contexts or explicit close() statements. Fix that.
Switch to nose2 for testing and mild doc cleanup
The orignal Nose project is deprecated and vaguely unmaintained. Switch to nose2, which is its successor.
Also fix some use of the deprecated assertEquals() method and autopep8 all py files.