Add more type hints to make mypy lint more

[kevinji]

I've just learned about python/mypy#4409 and in general, mypy doesn't type check a function if it has no annotations, so you need to often include an explicit return type to do so.

I also forgot that dict and list are type-checked starting from Python 3.9, so I removed the use of Dict and List.

[charlesetc]

Sounds great, thank you!