check-code: drop the check for whitespace around named parameters This check flags py3 annotations of named parameters, because `black` adds spaces around the assignment in this case. Since the chosen formatter has opinions (and pylint also wants the space in the case of annotations), drop the check so we can use py3 annotations.

rhg: add a config option to fall back immediately This is useful for debugging the behavior of the "default" `hg` in tests without having to manually substitute the fallback path.