Mypy support

[djmattyg007]

This package could benefit from type annotations, so that it can be type-checked when used in other software.

[zeroSteiner]

I'm starting to think this would be a good idea, but I don't really want to implement it until I drop support for more EOL'ed Python versions so I can use the newer syntax features. In the next major version, I'll drop 3.6 and 3.7.