Gets the directories under selfplay_dir that match YYYY-MM-DD-HH.

def get_hour_dirs(root=None):
    """Gets the directories under selfplay_dir that match YYYY-MM-DD-HH."""
    root = root or selfplay_dir()
    return list(filter(lambda s: re.match(r"\d{4}-\d{2}-\d{2}-\d{2}", s),
                       gfile.ListDirectory(root)))