Get iterator that returns (name, fs) in priority order.

def iterate_fs(self):
        # type: () -> Iterator[Tuple[Text, FS]]
        """Get iterator that returns (name, fs) in priority order.
        """
        if self._fs_sequence is None:
            self._fs_sequence = [
                (name, fs)
                for name, (_order, fs) in sorted(
                    self._filesystems.items(), key=itemgetter(1), reverse=True
                )
            ]
        return iter(self._fs_sequence)