Continuous Integration

Not much in the way of unit or integration tests yet, but I added a CI config that checks that Portmod installs properly, runs flake8, black --check and pytest (with one simple test added so that it passes).

I've also fixed the remaining issues that flake8 reports so that the test passes.

mypy could also be added (I've included it, though commented out), but unfortunately there seems to be an issue with mypy in the docker container. On my Gentoo machines, running mypy will only report errors in Portmod, but in the docker container (and also on my Arch machine for that matter) it reports a number of errors in the dependencies, meaning that even if we fixed the six errors that Portmod still throws, the test would fail unless we create stub files for all the dependencies in the tree (too many). A better solution would be to figure out why this doesn't happen on my Gentoo machines, but a bit of probing hasn't revealed anything. I think I'll just open an issue on the subject and until we fix it we can just run mypy manually.