Use `inspect.isawaitable` instead of custom `is_awaitable` helper function

[bartenra]

Since Python 3.5 there's an inspect.isawaitable function.

I believe the is_awaitable function in the code base should be removed. Instead, the function part of the standard library should be used.

I was having issues because I was using __getattr__ in a class and is_awaitable helper uses hasattr(value, "__await__"), resulting in a false positive.

[bartenra]

OK I just found this thread: #54