code: def _handle_websocket_headers(self, handler: WebSocketHandler) -> None:
          
          fields = ("Host", "Sec-Websocket-Key", "Sec-Websocket-Version")
          if not all(map(lambda f: handler.request.headers.get(f), fields)):
              raise ValueError("Missing/Invalid WebSocket headers")
docs: Verifies all invariant- and required headers
  
          If a header is missing or have an incorrect value ValueError will be
          raised