Function Annotations

[RedProkofiev]

Given we're moving to python 3 it would be really nice to take the time to add function annotations. It would give me and future maintainers a much better sense of types/type violations and what "should" happen from function to function.

This string/bytes issue #146 etc highlights this where part of the issue as a new maintainer was I wasn't necessarily sure (particularly with crypto etc) if something here SHOULD have been a string or a byte, and the whole process would've been self explanatory with what's essentially just a bit of extra documentation.

It'll also give us some insight as to when and where weird behaviour turns up if we encounter a similar issue.