Test on Python 3.7

Python 3.7 is available in Toolforge now and has a bunch of nice
additions compared to our current Python 3.5 (variable annotations,
added in Python 3.6, are probably the most immediately useful one), so
we’ll want to start using it soon.

(For some reason, the Trusty (14.04) environment on Travis CI doesn’t
have a Python “3.7”, only “3.7.0a2” and “3.7-dev”. See the table at [1]
for more details.)

[1]: https://docs.travis-ci.com/user/languages/python/#python-versions