Add type annotations

[sumezulike]

Adding typehints would greatly improve usability.

Without type annotations, IDE support is very limited and I often have to search through the code to figure out the exact types of a given attribute. Also static typechecking is obviously not possible, which I think is also pretty important for consistent code quality.

If that's an effort that would be appreciated, I would start by type-hinting the modules I currently use the most.