Generic type hints for `verify_all`'s `alist` and `formatter` parameters

[James-Ansley]

The verify_all function currently hints the parameters a_list as List[str] and formatter as Optional[Callable]. Perhaps better type hints would be (using 3.12 syntax but this could be done with the old-style generic type syntax):

def verify_all[T](
    ...
    alist: List[T],
    formatter: Optional[Callable[[T], str]] = None,
    ...
) -> None:

This would allow for any object to be passed in as the expected result with the formatter transforming the object into a string for approval — without type checkers complaining.

In fact, the default formatter for this function already calls str by default on objects from alist (see list_utils.py) when transforming the list to a string.

[JayBazuzi]

Agreed!! We DO want mypy to be stricter in the future.