docs: def access_token(self):
          """
          Retrieve and cache an access token to authenticate API calls.
          :return: An access token string.
          """
          if self._cached_access_token is not None:
              return self._cached_access_token
          resp = self._request(endpoint='access_token', data={'grant_type': 'client_credentials', 'scope': 'basic user'},
                               auth=(self.api_username, self.api_key))
          self._cached_access_token = resp['access_token']
          return self._cached_access_token