More strictness in mypy (WIP)

[prophile]

[PeterJCLaw]

:+1:

[coveralls]

Coverage increased (+0.0001%) to 99.954% when pulling 49c12efd022af81e864cd4a8293e33d10ad6a0bc on mypy-strictness into 507f3d71135ece7713c9b57c68523d7a033a119c on master.

[danpalmer]

@prophile I've fixed a few things here, but would appreciate a review as my approach might not be the best.

[prophile]

I don't think we should bypass the layering tests for type checking. Actually I'd claim that the layering tests are correctly exposing dependencies here: where we're implicitly depending on the shape of component A from component B that's still a dependency on A from B even if it's previously not been unmasked.