Add support for weak references to code objects. This will be used by an optimization in the incoming Python 3 JIT.

Patch by Reid Kleckner!
diff --git a/Misc/NEWS b/Misc/NEWS
index 03aa62d..6adc89d 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -19,6 +19,8 @@
   printed and Python exits.  Initialize the GIL before importing the site
   module.
 
+- Code objects now support weak references.
+
 Library
 -------