types: a typechecker

[emilyashley]

It might be nice to have type annotations and run a type checker setup for once we get into custom ad-ons to the base distro. This might take some discovery, I haven't gotten a chance to use python types before but I'd love to if we can!

If we decide to use mypy here's a cheatsheet and don't forget to include a py.typed file for packaging. (poetry may do this by default?)

[emilyashley]

We're addressing type annotations as a part of dev friendliness in #76 , we'll come back for more type checking explorations later!