Beef-up tests for collections ABCs.
diff --git a/Lib/_abcoll.py b/Lib/_abcoll.py
index ca82c41..eed8762 100644
--- a/Lib/_abcoll.py
+++ b/Lib/_abcoll.py
@@ -301,7 +301,7 @@
         """Return the popped value.  Raise KeyError if empty."""
         it = iter(self)
         try:
-            value = it.__next__()
+            value = next(it)
         except StopIteration:
             raise KeyError
         self.discard(value)