docs: def list_device_data_sources(self, device_rid):
          """
              List data sources of a portal device with rid 'device_rid'.
  
              http://docs.exosite.com/portals/#list-device-data-source
          """
          headers = {
                  'User-Agent': self.user_agent(),
          }
          headers.update(self.headers())
  
          r = requests.get(   self.portals_url()+'/devices/'+device_rid+'/data-sources', 
                      headers=headers, auth=self.auth())
          if HTTP_STATUS.OK == r.status_code:
              return r.json()
          else:
              print("Something went wrong: <{0}>: {1}".format(
                          r.status_code, r.reason))
              return None