Fix a reference leak found by Georg, when compiling a class nested in another class.

Test is run with "regrtest.py -R:: test_compile"

Backport of r62015

Lib/test/test_compile.py

@@ -398,6 +398,10 @@ if 1:
         del d[..., ...]
         self.assertEqual((Ellipsis, Ellipsis) in d, False)
 
+    def test_nested_classes(self):
+        # Verify that it does not leak
+        compile("class A:\n    class B: pass", 'tmp', 'exec')
+
 def test_main():
     test_support.run_unittest(TestSpecifics)