Use correct PyGC_Head size in tests for issue #25421.

diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index e6c17a2d35f760a750f337726e97070498ad1865..4afb78059938b63f639dab7fd850db31396bff68 100644 (file)
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -760,7 +760,7 @@ class SizeofTest(unittest.TestCase):
     def check_slots(self, obj, base, extra):
         expected = sys.getsizeof(base) + struct.calcsize(extra)
         if gc.is_tracked(obj) and not gc.is_tracked(base):
-            expected += struct.calcsize('3P')  # PyGC_Head
+            expected += self.gc_headsize
         self.assertEqual(sys.getsizeof(obj), expected)
 
     def test_slots(self):