Upgrade linter versions to the newest versions that still install under both Python 2 and Python 3.7

[obi1kenobi]

We picked up some bugfixes along the way, so I fixed the lint they detected, and also seemingly a bug in pylint, which I reported: pylint-dev/pylint#3039

[obi1kenobi]

We run linting under Python 3.6, but unfortunately the way that pipenv is currently set up requires the linters to successfully install under all environments, and pylint==1.9.5 is marked as not supporting Python 3.7. Similarly, pylint>=2 are Python 3-only, so we have to use pylint==1.9.4 as the closest version that isn't blacklisting Python 3.7. I suspect that version doesn't actually support 3.7, but since the development environment (and by extension the lint environment) is using 3.6, that doesn't really matter here.

TL;DR: My solution is ugly, but it works well enough to not be worth spending additional time on at the moment.

[pmantica1]

Shouldn't this be "python_requires > 3.7" ? (That being the reason why it doesn't work with python2.)

[obi1kenobi]

No. pylint>=2 is python_requires >= 3.3, and pylint==1.9.5 is python_requires < 3.7 -- it explicitly does not support Python 3.7 and is 3.6-and-below only.

[obi1kenobi]

Updated the PR title to clarify.

[pmantica1]

Ok got it. Misunderstood what this meant.