inline type annotations for datastructures

[davidism]

Currently we have a bunch of .pyi files with the annotations, because Python <= 3.8 didn't support generic list and dict subclassing. Now that we support >= 3.9, we should be able to move all the annotations into the Python code and remove the stubs.

[sgaynetdinov]

@davidism Hi, is this task available?

[davidism]

I'm currently most of the way through it, thanks for offering though!