Simplify FactCache.copy()

Also fix the bug (missing from six import iteritems) I introduced in
823677b490.

lib/ansible/plugins/cache/__init__.py

@@ -60,7 +60,7 @@ class FactCache(MutableMapping):
 
     def copy(self):
         """ Return a primitive copy of the keys and values from the cache. """
-        return dict([(k, v) for (k, v) in iteritems(self)])
+        return dict(self)
 
     def keys(self):
         return self._plugin.keys()