Consider migrating from `mypy` to `pyright`

Pyright is faster, more robust, and more "modern" compared with `mypy`. We should probably consider using it for these reasons. Or, if we find a strong reason not to use it, we can close this as "won't do".

https://github.com/microsoft/pyright/blob/main/docs/mypy-comparison.md

Probably should first deliver this one:

- #11