Lint using Mypy

[danieleades]

i'm playing around using mypy to do some type checking on this branch

by refactoring some of the imports, i've got the number of type errors down from ~60 to 6.

this branch depends on #154

It would also be nice to add type annotations throughout the codebase, since I can see needs is already targeting python >= 3.5. This can be adopted incrementally.

[danieleades]

i think i'll close this.

in its current state, mypy can't be used effectively on this repo.

hopefully can revisit later