Fix import that was breaking py3k

--HG--
branch : distribute
extra : rebase_source : 76bf8f9213536189bce76a41e798c44c5f468cbd

setuptools/extension.py

 from distutils.core import Extension as _Extension
-from dist import _get_unpatched
+from setuptools.dist import _get_unpatched
 _Extension = _get_unpatched(_Extension)
 
 try: