Improve type checking

[ValerianRey]

Currently, there are a few issues with the way we do type-checking:

• We use pyright in vscode but mypy in ci
• We have about 26 errors in the src reported by pyright in standard mode (but 0 reported by mypy in default mode)

I think we should settle for a type checker that:

• Works in vscode and in ci
• Can be strict (note that mypy can be made much stricter than what we use, by changing its options)
• Is quite easy to configure and maintain

I think we could maybe also try ty, which is much faster than mypy and pyright, and seems to work both in CI and vscode.

I really don't know what would be best (honestly I think they would all be ok for our use-cases, if we manage to configure them correctly), but what we're doing right now is a bit frustrating. So we need to:

• Choose a good enough type checker
• Configure it properly
• Fix the typing errors we still have (at least in src)

[PierreQuinton]

We need to have the same checker in the interpreter in VSCode and in the CI. I don't mind (Would appreciate?) having something stricter. Let's try ty locally and in a PR to see how this goes.