[generation] fix type hint (#35725)

fix type hint

src/transformers/generation/streamers.py

@@ -284,7 +284,7 @@ class AsyncTextIteratorStreamer(TextStreamer):
     """
 
     def __init__(
-        self, tokenizer: "AutoTokenizer", skip_prompt: bool = False, timeout: float | None = None, **decode_kwargs
+        self, tokenizer: "AutoTokenizer", skip_prompt: bool = False, timeout: Optional[float] = None, **decode_kwargs
     ):
         super().__init__(tokenizer, skip_prompt, **decode_kwargs)
         self.text_queue = asyncio.Queue()