Clean up nanny WorkerProcess.kill

[gjoseph92]

I'm hoping that this resolves #6971, though it may not.

This fixes a couple things:

1. When the subprocess exits, it'll call mark_stopped, which sets child_stop_q to None among other things. So between self.child_stop_q.put and self.child_stop_q.close, self.child_stop_q could have become None, since there's an await in between. That caused the AttributeError: 'NoneType' object has no attribute 'close'.
2. Once that's resolved, we'd get to try to call process.join, after process.close has been called (in mark_stopped). That will cause join to raise a ValueError. This is fine—it means we can just exit—but we should catch that. I went for a string match on the ValueError since ValueError feels like too generic of an exception type to just ignore IMO.

I have no idea how to test this. The condition we're looking for is the subprocess exiting in between an await sleep(0). That's extremely hard to trigger. (I'm also not sure it still needs to be there??)

Closes #6971

• Tests added / passed
• Passes pre-commit run --all-files

[fjetter]

FWIW I started cleaning up all the queues in #6655 but never got around to finish it. I removed the sleep(0) IIRC

[fjetter]

I don't love the ValueError handling but the rest is a straight forward change so if there is a chance this will help, that's great.