Fix server shutdown for Python 3.10-3.12

[dlqqq]

Description

• Closes Cannot shut down asyncio Queues in Python <3.13 #64
• Replaces calls to asyncio.Queue.shutdown() with asyncio.Queue.put_nowait(None).
• When either queue in YRoom and YRoomFileAPI receives an item of value None, the background task watching that queue will shut down.
• Adds docstrings that clearly define this behavior for future developers.

Testing instructions

• Create a new environment
• Use python=3.10 and jupyterlab=4.4 (our extension still requires JL >=4.4)
• Run jlpm dev:install
• Start the server & open a text & notebook file
• Verify that the server shuts down gracefully

Future work

• Python 3.9 support still needs more investigation. I've opened a new issue for this: Fix Python 3.9 support (?) #74.

[dlqqq]

These lines were never called, since the _process_scheduled_saves() task always waits on the content to be loaded. Since the server won't stop until all of the background tasks are stopped, the content is always loaded before shutdown and we don't need to check its status.

We can add a way to cancel the initial content loading while the server is shutting down in a future PR. That change would only speed up server shutdown by 1-100 ms, so I don't think it's a priority for us right now.

[Zsailer]

Thanks, @dlqqq ! Looks great. Pulled this PR down and tried it on Python 3.10. Works great.