gh-79033: Try to fix asyncio.Server.wait_closed() again

[gvanrossum]

I identified the condition that wait_closed() is intended to wait for: the server is closed and there are no more active connections.

When this condition first becomes true, _wakeup() is called (either from close() or from _detach()) and it sets _waiters to None. So we just check for self._waiters is None; if it's not None, we know we have to wait, and do so.

A problem was that the new test introduced in 3.12 explicitly tested that wait_closed() returns immediately when the server is not closed but there are currently no active connections. This was a mistake (probably a misunderstanding of the intended semantics). I've fixed the test, and added a separate test that checks exactly for this scenario.

I also fixed an oddity where in _wakeup() the result of the waiter was set to the waiter itself. This result is not used anywhere and I changed this to None, to avoid a GC cycle.

• Issue: Counter-intuitive behavior of Server.close() / wait_closed() #79033

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