Add mild type annotations

[mrocklin]

We should investigate adding mild type annotations to this project. By this I mean simple types like str and int that aren't contentious, and nothing complex.

We might then investigate the value of adding mypy to CI.

I think that this would be a good first step before adding typing annotations to the core dask project. I think that it makes sense to start here because:

1. We're more tied to Python 3 due to the async-await syntax
2. We're slightly less core

[jrbourbeau]

It's been a while since this issue was originally opened and since then some type annotations have made there way into the distributed code base. If we're going to have type annotations (which I think there's interest in) then we should consider adding a mypy check in our CI to make sure they are correct (maybe in our existing pre-commit check?)

@crusaderky you and I have talked about this a few times offline. IIRC you mentioned, since mypy will effectively ignore non-annotated functions, adding an initial mypy check may be relatively straightforward. If that's the case, is this something you're interested in taking on?