Add typings to the codebase

Typings have been supported since python 3.5 and could be applied to the codebase. This would help readability a lot, help with IDE syntax, etc

### Objective

Adding typings to the entirety of the codebase would be the goal although that's obviously a very long way in the future.

I'm happy to start and try and add these a module at a time.

The issue names no specific files, tests, or entry points. Start by selecting one module from the codebase, inspect its existing interfaces, and determine how its annotations can be checked; done means that module has useful typings without changing its behavior.

Written by the indexing model from the issue text.