Establish unified coding conventions and style guide

As I was reviewing a merge request again I caught myself thinking stuff like "this looks really weird…" before having the realisation that our coding styles began to diverge somewhere. (Disclaimer: This was right after I woke up so take that evaluation itself with a grain of salt). And thus it occurred to me that establishing (or at the least, adopting) would be more than a good idea at this point.

PEP8 is probably a good starting point for us—given its nature of being designed for, and evolving with, Python itself—but is not something we should solely rely on.

On top of that we really should establish some solid work patterns and procedures in addition to coding conventions for the sake of quality/our sanity. For example, there's several features of GitLab (or—for that matter—GitHub, if we were working there instead) that don't work at all or give erroneous output because of our own inconsistency and ignorance. Of course, some of these patterns we have yet to develop should already be documented in the currently empty contribution guidelines we have…