Call plugin get_attribute_hook() also for attributes that trigger __getattr__()

[ilevkivskyi]

Currently, get_attribute_hook() is only called for attributes that are already present in the class symbol table. However, the typical use case is opposite -- dynamically added attributes. Therefore, the current behavior creates some complications while developing plugins, like every access to dynamic attribute first requires somehow placing it into the class symbol table using other hooks. I propose to widen the scope of get_attribute_hook() to at least be called for classes with __getattr__().