SF patch #667730: More DictMixin

* Adds missing pop() methods to weakref.py
* Expands test suite to broaden coverage of objects with
  a mapping interface.

Contributed by Sebastien Keim.
diff --git a/Misc/ACKS b/Misc/ACKS
index f4ecd10..abc1c75 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -281,6 +281,7 @@
 Tamito Kajiyama
 Jacob Kaplan-Moss
 Lou Kates
+Sebastien Keim
 Randall Kern
 Robert Kern
 Magnus Kessler