Yield the services eventable variables.

        Yields:
            `tuple`: a tuple of (variable name, data type).

def iter_event_vars(self):
        """Yield the services eventable variables.

        Yields:
            `tuple`: a tuple of (variable name, data type).
        """

        # pylint: disable=invalid-name
        ns = '{urn:schemas-upnp-org:service-1-0}'
        scpd_body = requests.get(self.base_url + self.scpd_url).text
        tree = XML.fromstring(scpd_body.encode('utf-8'))
        # parse the state variables to get the relevant variable types
        statevars = tree.findall('{}stateVariable'.format(ns))
        for state in statevars:
            # We are only interested if 'sendEvents' is 'yes', i.e this
            # is an eventable variable
            if state.attrib['sendEvents'] == "yes":
                name = state.findtext('{}name'.format(ns))
                vartype = state.findtext('{}dataType'.format(ns))
                yield (name, vartype)