Extend mypy coverage to Python demos

[schnellerhase]

Currently the Python demos are not covered with the mypy check, this should be included in the type checks.

[risa-hirsh]

Hi, is this something I could work on? I don't see this labeled as "good first issue" and I'm new to contributing, would this still be appropriate for me to help with?

[schnellerhase]

Always, thanks! The checks in the CI can be found here https://github.com/FEniCS/basix/blob/main/.github/workflows/pythonapp.yml#L38-L42.