Fix syntax error under Python 3.2.

django/utils/functional.py

@@ -25,9 +25,9 @@ def memoize(func, cache, num_args):
 
     Only the first num_args are considered when creating the key.
     """
-    warnings.warn(u"memoize wrapper is deprecated and will be removed in "
-                  u"Django 1.9. Use django.utils.lru_cache instead.",
-                  PendingDeprecationWarning, 2)
+    warnings.warn("memoize wrapper is deprecated and will be removed in "
+                  "Django 1.9. Use django.utils.lru_cache instead.",
+                  PendingDeprecationWarning, stacklevel=2)
 
     @wraps(func)
     def wrapper(*args):