Suggest type checking tool and configuration, and maybe add to tox?

[handrews]

I decided to try to run mypy because I keep forgetting / messing up type annotations, and it returned 519 errors in the package code and 8 in the tests. Looking at the errors a lot of them are clearly over-zealous attempts to make Python into Java or C#. For example, it gets very confused by the mix of JSON and JSONSchema objects in a JSON Schema document and can't work out that evaluate() is only called on JSONSchema instances.

I am assuming that either there is a specific configuration or a different tool intended for use? I'd be happy to add something to the tox configuration if there is a type checking setup that is expected to pass.