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.

tests/protocols/test_websocket.py

@@ -9,7 +9,6 @@
 import websockets
 import websockets.client
 import websockets.exceptions
-from typing_extensions import TypedDict
 from websockets.extensions.permessage_deflate import ClientPerMessageDeflateFactory
 from websockets.typing import Subprotocol
 
@@ -777,7 +776,7 @@ async def websocket_session(url: str):
     assert disconnected_message == {"type": "websocket.disconnect", "code": 1006}
 
 
-class EmptyDict(TypedDict): ...
+class EmptyDict(typing.TypedDict): ...
 
 
 async def test_server_reject_connection_with_response(