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  Because the "old" Interface package allowed the import of the Attribute
  class directly from the Interface package, and some packages
  (such as CMF) expect a class back from "from Interface import Attribute":

  - Renamed Attribute.py to _Attribute.py and did the necessary
    housekeeping to make sure all the other parts of the Interface
    package knows about the change.

  - Changed __init__.py to import Attribute from _Attribute

New features and changes:

  - Removed iclass.py  (it was no longer used).

  - Gave the base interface class (Interface._InterfaceClass) a __hash__
    method as ExtensionClass instances do not hash like normal
    instances, and we require that interfaces be hashable in order to
    store them as dictionary keys and whatnot in registries.