Removed the support of writing async __new__

[francis-clairicia]

Why remove async def __new__?

When I wrote the first version of this library, it was mostly to prove that something like this was possible. But in practical use:

1. There is no need to perform asynchronous operations on object allocation.
2. This can lead to confusion with other base classes.
3. It is hard to maintain such a feature that involves the creation of the object itself.