Add python 3.5 and 3.6 to CI

[WhyNotHugo]

Fun fact: flake was set to run with python 2.7 and 3.5, but 3.5 was not in the build matrix.

Also did some simplification, since we could remove some conditionals, variable, etc.

[WhyNotHugo]

Note that current master of pip has dropped support for python2.6, so it might be about time to drop that too.

[WhyNotHugo]

The virtualenv needs to be in a different directory, otherwise some tests will fail on OSX. Just leaving a comment here for posterity's sake.

[untitaker]

Thanks, but in your PR TOXENV serves no purpose and I also think the stylechecks are not executed.

[WhyNotHugo]

Thanks, but in your PR TOXENV serves no purpose and I also think the stylechecks are not executed.

Oops! Fixed now!

[untitaker]

Thanks!