Get mypy passing

[2e0byo]

We have at least one type error; get_distinct should return a set, not a list. (Clearly none of the distinct set api is being used.) Mypy doesn't currently run, and won't until we have types for tidalapi. So to get mypy passing we need to add type hints to tidalapi (we can use a stub for now so no need to wait on tidalapi), and fix some internal problems. I'd love to know, for instance, why self.backend is thought not to have a .session.

As an added benefit modern IDEs will be better at auto-completing.