Port 2.7 fix for sporadic failure in test_weakset.

Lib/_weakrefset.py

@@ -114,9 +114,6 @@ class WeakSet:
     def update(self, other):
         if self._pending_removals:
             self._commit_removals()
-        if isinstance(other, self.__class__):
-            self.data.update(other.data)
-        else:
         for element in other:
             self.add(element)