Avoid test dependency on typing_extensions

[carlwgeorge]

test_websocket.py imports TypedDict from 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. I think this currently goes unnoticed because the CI installs mypy which has its own dependency on typing_extensions, but if you run the tests without mypy installed it becomes clear that typing_extensions is currently required.

I'd like to send a pull request to switch test_websocket.py to import TypedDict from the standard library instead of from typing_extensions.

[Kludex]

Sure. PR welcome. Thanks!