Consider migrating from mypy to pyright

[aaronsteers]

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:

• Achieve baseline mypy compliance across the repository #11