Get the checksum stored in :attr:`checksum_file` (a string or :data:`None`).

def old_checksum(self):
        """Get the checksum stored in :attr:`checksum_file` (a string or :data:`None`)."""
        if self.context.is_file(self.checksum_file):
            logger.debug("Reading saved checksum from %s ..", format_path(self.checksum_file))
            checksum = self.context.read_file(self.checksum_file).decode('ascii')
            logger.debug("Saved checksum is %s.", checksum)
            return checksum