Add test coverage to Py3.7, Py3.8, Py3.9

[javulticat]

This PR adds test coverage to Py3.7, Py3.8, and Py3.9. Related issue is #60. It also fixes the Py3.4 test, related issue being #61.

Looking at the output on Travis, it appears the test suite is working as expected with these changes.

However, I do have a few outstanding questions:

1. It looks like Travis is using the config found in .travis.yml. So, what is the travis.yml file in this repo doing, if anything? Is that file still needed or can it be removed?
2. Relatedly, it looks like travis.yml (which, again, AFAICT seems to not be doing anything) includes running isort, while .travis.yml (which appears to be what Travis is using) does not. Should we add the isort check to .travis.yml?
3. By now, Py2.7, Py3.4 and Py3.5 have all reached EOL (on 2020-01-01, 2019-03-18, and 2020-09-05, respectively). Is test coverage still needed for these versions, in your opinion, or should it be removed similar to how coverage for Py3.3 was removed previously after its EOL?

Thanks!