Avoid test dependency on typing_extensions

TypedDict was added to the standard library typing module in Python 3.8.
Since this project already requires at least Python 3.9, we can rely on
this being available.

Author: carlwgeorge

tests/protocols/test_websocket.py

@@ -9,7 +9,7 @@
 import websockets
 import websockets.client
 import websockets.exceptions
-from typing_extensions import TypedDict
+from typing import TypedDict
 from websockets.extensions.permessage_deflate import ClientPerMessageDeflateFactory
 from websockets.typing import Subprotocol