Allow coroutines on add_listener.

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The current code that process a notification uses the method `call_soon` which expects a callable, why not to use `create_task` (or `ensure_future` for compatibility reasons) after checking if the callback is a coroutine?

There are different ways to do this, either just checking if the callback is a coroutine as described in [here](https://stackoverflow.com/questions/36076619/test-if-function-or-method-is-normal-or-asynchronous) for example. Another way is just to add an extra argument or a different method to register coroutines.

Would it make sense to add a context? I am currently passing extra arguments to the callback via `functools.partial`.

Let me know if something like this would be accepted I can create a PR with the implementation.

Start at the add_listener entry point and the notification processing that currently calls call_soon. Compare create_task and ensure_future for coroutine callbacks, including the existing functools.partial usage for extra arguments. Done should include an agreed callback API and compatibility behavior, with tests covering regular and coroutine listeners.

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