update tox/travis to check code coverage on py3

Coverage is combined between py2 and py3 for an aggregate coverage
metric. This means we can stop putting "no cover" gates around py3 code
and ignoring whether it's ever tested.

This is the best way I could find to do this based on several resources [1,2]. I'd be ecstatic if someone has a cleaner way. The only negative I see of this approach is running the tests one final time to dump xunit and coverage.xml files.

Fixes #1470.

[1] http://blog.ionelmc.ro/2014/05/25/python-packaging/
[2] https://github.com/tuskar/python-tuskarclient/blob/master/tox.ini