code: def update_notes(self, **kwargs):
          
          for key, val in iteritems(kwargs):
              setattr(self, key, val)
          url = urljoin(self._url, '/notes')
          self.put(url)
docs: Updates the notes on the subscription without generating a change
          This endpoint also allows you to update custom fields:
  
              `
                  sub.custom_fields[0].value = 'A new value'
                  sub.update_notes()
              `