[1.5.x] Fixed URL for six inventory.

Backport of 749c2e4 from master.

docs/conf.py

@@ -94,7 +94,7 @@ pygments_style = 'trac'
 intersphinx_mapping = {
     'python': ('http://docs.python.org/2.7', None),
     'sphinx': ('http://sphinx.pocoo.org/', None),
-    'six': ('http://packages.python.org/six/', None),
+    'six': ('http://pythonhosted.org/six/', None),
     'simplejson': ('http://simplejson.readthedocs.org/en/latest/', None),
 }