Revert SF #1615701 (rev 53655): dict.update() does *not* call __getitem__() or
keys() if subclassed.  This is to remain consistent with 2.5.

See discussion here:
  http://mail.python.org/pipermail/python-dev/2007-April/072565.html
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index bc3cd53..af0d6f3 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -1352,7 +1352,7 @@
 		return -1;
 	}
 	mp = (dictobject*)a;
-	if (PyDict_CheckExact(b)) {
+	if (PyDict_Check(b)) {
 		other = (dictobject*)b;
 		if (other == mp || other->ma_used == 0)
 			/* a.update(a) or a.update({}); nothing to do */