Return the expiration timestamp for the session cookie.

        :rtype: datetime

def _cookie_expiration(self):
        """Return the expiration timestamp for the session cookie.

        :rtype: datetime

        """
        value = (datetime.datetime.utcnow() +
                 datetime.timedelta(seconds=self._session_duration))
        LOGGER.debug('Cookie expires: %s', value.isoformat())
        return value