Add basic support for typechecking

Static typechecking can help make IDEs and editors smarter, uncover hidden bugs, and may help people read and understand the code better.

This merge request:

1. enables static typechecking via mypy (see also extra/mypy);

2. fixes a couple of typing issues, which would interfere with type checking;

3. adds a CI step that will fail when a commit introduces typing errors.

The basic MyPy configuration this MR introduces is pretty lenient: it deliberately leaves two helpful typing rules switched off (disallow_untyped_calls and disallow_untyped_defs). That allows any piece of code to opt into typing annotations (instead of making them a hard requirement). Should the project team decide to go the stricter route, I’ll be there and willing to do follow-up MRs to add type declarations to the rest of the code.

This MR requires Python 3.10 or newer (compared to current master, which requires 3.9 or newer).
Shouldn’t be an issue though, given that namcap runs on system Python anyway, which is at 3.11 3.12.