Log Message:
Updated py-ZopeInterface to 4.3.1.

Add missing dependency.

4.3.1 (2016-08-31)
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- Support Components subclasses that are not hashable.
  (https://github.com/zopefoundation/zope.interface/issues/53)

4.3.0 (2016-08-31)
------------------

- Add the ability to sort the objects returned by ``implementedBy``.
  This is compatible with the way interface classes sort so they can
  be used together in ordered containers like BTrees.
  (https://github.com/zopefoundation/zope.interface/issues/42)

- Make ``setuptools`` a hard dependency of ``setup.py``.
  (https://github.com/zopefoundation/zope.interface/issues/13)

- Change a linear algorithm (O(n)) in ``Components.registerUtility`` and
  ``Components.unregisterUtility`` into a dictionary lookup (O(1)) for
  hashable components. This substantially improves the time taken to
  manipulate utilities in large registries at the cost of some
  additional memory usage. \ 
(https://github.com/zopefoundation/zope.interface/issues/46)