Gitlab CI and linting

[Kai Willadsen <kai willadsen gmail com>]

Hi all,

So the whole move-to-Gitlab thing has gone fine from a "it seems to
work" standpoint, but there's definitely going to be some teething
issues while we figure out how everything is actually going to work.

I've just added a rudimentary CI setup to run our very limited set of
tests and, more importantly, a basic linting setup. This isn't
currently running, but hopefully soon this will provide style, etc.
checking on merge requests.

In the meantime, I've pushed lots of cleanups such that a boring
`flake8 meld/` passes; the list of required plugins is in
dev-requirements.txt. When I sort the CI out, I'm intending on making
this a mandatory check.

cheers,
Kai