Get mypy passing with check_untyped_defs set to True

[bmw]

#29 needs to be resolved before this.

[github-actions]

We've made a lot of changes to Josepy since this issue was opened. If you still have this issue with an up-to-date version of Josepy, can you please add a comment letting us know? This helps us to better see what issues are still affecting our users. If there is no activity in the next 30 days, this issue will be automatically closed.