Fix typo in __slots__ of ImmutableSet.
diff --git a/Lib/sets.py b/Lib/sets.py
index 1072dd2..2d44c28 100644
--- a/Lib/sets.py
+++ b/Lib/sets.py
@@ -273,7 +273,7 @@
 class ImmutableSet(BaseSet):
     """Immutable set class."""
 
-    __slots__ = ['_hash']
+    __slots__ = ['_hashcode']
 
     # BaseSet + hashing