Generate the given template with the given arguments.

        We return the generated byte string (in utf8). To generate and
        write a template as a response, use render() above.

def render_string(self, template_name: str, **kwargs: Any) -> bytes:
        """Generate the given template with the given arguments.

        We return the generated byte string (in utf8). To generate and
        write a template as a response, use render() above.
        """
        # If no template_path is specified, use the path of the calling file
        template_path = self.get_template_path()
        if not template_path:
            frame = sys._getframe(0)
            web_file = frame.f_code.co_filename
            while frame.f_code.co_filename == web_file:
                frame = frame.f_back
            assert frame.f_code.co_filename is not None
            template_path = os.path.dirname(frame.f_code.co_filename)
        with RequestHandler._template_loader_lock:
            if template_path not in RequestHandler._template_loaders:
                loader = self.create_template_loader(template_path)
                RequestHandler._template_loaders[template_path] = loader
            else:
                loader = RequestHandler._template_loaders[template_path]
        t = loader.load(template_name)
        namespace = self.get_template_namespace()
        namespace.update(kwargs)
        return t.generate(**namespace)