update tests to check coverage of py3 code instead of setting "no cover"

[mmerickel]

It's possible to run coverage on multiple targets and combine their results into a final analysis. We should try to do this to properly test coverage on py2 and py3.

http://nedbatchelder.com/code/coverage/cmd.html#cmd-combining
http://stackoverflow.com/questions/19274492/how-calculate-the-global-coverage

Related: #979

[gcarothers]

How does this play with tox setup on travis? Doesn't each one of the builds get executed in it's own sandbox? Or is this only for local execution?

[gcarothers]

Heh, I see you in fact already ran into this in 11ba5a5