Feature/implement mypy type checking

[YPCrumble]

This PR adds initial MyPy type checks per discussion in #393.

[medmunds]

Thanks for this! I want to poke around with it a little, and then will probably have some questions.

The separate GH workflow for mypy is fine for getting things going, but we'll eventually want to move running mypy into tox.ini to simplify local runs. (I can do that later if you're not familiar with tox.)

Is there a reason docs needs to be turned into a module? (We deliberately don't support importing from docs.)

[YPCrumble]

Sounds good @medmunds. No need to make docs a module, I adjusted how we're excluding it and removed the __init__.py file.

[YPCrumble]

Thank you for the review @medmunds !!! I wanted to respond to your thoughtful comments and also let you know I'm a little busy over the coming months until the new year so won't be able to get back to this...feel free to take over or I will come back to it later!