code: def add_header(self, header, value):
          # type: (str, str) -> None
          
          warnings.warn("Private attribute _client.add_header is deprecated. Use config.headers instead.",
                        DeprecationWarning)
          self.config.headers[header] = value
docs: Add a persistent header - this header will be applied to all
          requests sent during the current client session.
  
          .. deprecated:: 0.5.0
             Use config.headers instead
  
          :param str header: The header name.
          :param str value: The header value.