Fix mypy typing errors

MR !6 (merged) fixes a lot of pre-commit errors but left some mypy errors. This commit fixes them: most of the fixes do not change the semantics (they are just here to calm mypy).

I am uneasy with api.unwrap: in Python, x.v raises an exception if x is None, and sometimes we want that (if x is None, then there is a problem and we want an exception to fail in this case). When x is of type Optional[T], mypy requires to check that x is not None: that is the point of the function api.unwrap.unwrap, but that breaks Python's tradition of "Ask forgiveness not permission".