redis: add ping to PubSub (#3583)

third_party/2and3/redis/client.pyi

@@ -317,6 +317,7 @@ class PubSub:
     def get_message(self, ignore_subscribe_messages: bool = ..., timeout: float = ...) -> Optional[Dict[str, Any]]: ...
     def handle_message(self, response, ignore_subscribe_messages: bool = ...) -> Optional[Dict[str, Any]]: ...
     def run_in_thread(self, sleep_time=...): ...
+    def ping(self, message: Optional[_Str] = ...) -> None: ...
 
 class BasePipeline:
     UNWATCH_COMMANDS: Any