IsCoroutineArgMeta.__setattr__ must override the right method
IsCoroutineArgMeta.__setattr__ must override the right method

We define __setattr__ as a wrapper (overriden) method in the metaclass,
the previous implementation didn't really comply with how super()
methods are found: we must see if there's a __setattr__ attribute is the
class and then fallback to the parent

Martiusweb/asynctest