code: def set_tunnel(self, host, port=None, headers=None):
          
          self._tunnel_host = host
          self._tunnel_port = port
          if headers:
              self._tunnel_headers = headers
          else:
              self._tunnel_headers.clear()
docs: Sets up the host and the port for the HTTP CONNECT Tunnelling.
  
          The headers argument should be a mapping of extra HTTP headers
          to send with the CONNECT request.