Run tests again python 3.7

[non-Jedi]

3.7 was released in late June. We should run tests against it like we do all the other python 3 versions. Is 3.4 deprecated yet? If so, we should remove testing for it.

[slipeer]

Seems that 3.4 not deprecated yet https://www.python.org/dev/peps/pep-0429/

[non-Jedi]

Per #262, we'll probably be removing support for 3.4 anyway unless some new objections come through.