Type hints and changes to class inheritance

[bmos]

This changes from the MixIn pattern to direct inheritance (since it only inherits from one class) and adds type hints to pretty much every arg and return. Since the MixIn was not combining many classes, this should work just as well and it's better supported by the 'magic' of modern IDEs.

It passes the limited pytest implementation, but I'm not so familiar with this module and it's quite likely that this could break things. So I'd advise against merging it without doing at least some basic tests yourself.

I tried to keep most of these commits standalone so it shouldn't be too hard to skip some of them if they're too opinionated, although once I split it into multiple files it probably gets harder to piecemeal it.

[lordjabez]

really appreciate the contributions @bmos! I inherited this project a few years back and haven't had the time to give it the love it deserves.

Will do some testing when I get time (hopefully in the next few weeks).