stubgen does not preserv @abstractmethod decorator

[dmoisset]

When a method is decorated by abc.abstractmethod stubgen does not preserved (this works for builtindecorators like classmethod, staticmethod or property)

Comes from #3169 (comment)

[hoefling]

Just passing by here, but I think this was added by #6256 already, so this issue can be closed...

[ilevkivskyi]

@hoefling Thanks, indeed.