GH-132554: "Virtual" iterators (GH-132555)

* FOR_ITER now pushes either the iterator and NULL or leaves the iterable and pushes tagged zero

* NEXT_ITER uses the tagged int as the index into the sequence or, if TOS is NULL, iterates as before.

Lib/test/test_list.py

@@ -365,5 +365,20 @@ class ListTest(list_tests.CommonTest):
         rc, _, _ = assert_python_ok("-c", code)
         self.assertEqual(rc, 0)
 
+    def test_list_overwrite_local(self):
+        """Test that overwriting the last reference to the
+           iterable doesn't prematurely free the iterable"""
+
+        def foo(x):
+            self.assertEqual(sys.getrefcount(x), 1)
+            r = 0
+            for i in x:
+                r += i
+                x = None
+            return r
+
+        self.assertEqual(foo(list(range(10))), 45)
+
+
 if __name__ == "__main__":
     unittest.main()