gh-74929: Remove undesirable DECREF in PEP 667 implementation (#118583)

With tests.

Lib/test/test_frame.py

@@ -364,6 +364,21 @@ class TestFrameLocals(unittest.TestCase):
             c = 0
         f()
 
+    def test_local_objects(self):
+        o = object()
+        k = '.'.join(['a', 'b', 'c'])
+        f_locals = sys._getframe().f_locals
+        f_locals['o'] = f_locals['k']
+        self.assertEqual(o, 'a.b.c')
+
+    def test_update_with_self(self):
+        def f():
+            f_locals = sys._getframe().f_locals
+            f_locals.update(f_locals)
+            f_locals.update(f_locals)
+            f_locals.update(f_locals)
+        f()
+
     def test_repr(self):
         x = 1
         # Introduce a reference cycle

Objects/frameobject.c

@@ -171,7 +171,6 @@ framelocalsproxy_setitem(PyObject *self, PyObject *key, PyObject *value)
             } else if (value != oldvalue) {
                 Py_XSETREF(fast[i], Py_NewRef(value));
             }
-            Py_XDECREF(value);
             return 0;
         }
     }