WIP: Improve typing

[defnull]

Add typing annotations compatible with all supported Python releases (>=3.8). It is not a target to annotate every single thing, just public APIs.

[adamtheturtle]

I appreciate that this is WIP, but just in case it is useful - to be useful for users, add a py.typed file (https://typing.readthedocs.io/en/latest/guides/libraries.html#marking-a-package-as-providing-type-information).