Return an RFC 3339-compliant timestamp.

        Returns:
            (str): Timestamp string according to RFC 3339 spec.

def rfc3339(self):
        """Return an RFC 3339-compliant timestamp.

        Returns:
            (str): Timestamp string according to RFC 3339 spec.
        """
        if self._nanosecond == 0:
            return to_rfc3339(self)
        nanos = str(self._nanosecond).rjust(9, '0').rstrip("0")
        return "{}.{}Z".format(self.strftime(_RFC3339_NO_FRACTION), nanos)