Get the SHA1 digest of the contents of :attr:`filename` (a string).

def new_checksum(self):
        """Get the SHA1 digest of the contents of :attr:`filename` (a string)."""
        if self.context.is_file(self.filename):
            friendly_name = format_path(self.filename)
            logger.debug("Calculating SHA1 of %s ..", friendly_name)
            context = hashlib.sha1()
            context.update(self.context.read_file(self.filename))
            checksum = context.hexdigest()
            logger.debug("The SHA1 digest of %s is %s.", friendly_name, checksum)
            return checksum