Target.is_class should use inspect.isclass

[abeinstein]

Is there any reason why Target.is_class doesn't invoke the inspect module's isclass method? The current implementation can break if the doubled object overrides __eq__.

This revealed a bug in my __eq__ override (since I should immediately return False if the other object is not an instance of self), but it seems cleaner to just inspect the object directly instead of invoking __eq__.