bpo-33418: Add tp_clear for function object (GH-8058)

Without tp_clear, GC can't break cyclic reference.
It will cause memory leak when cyclic reference is
created intentionally.

Misc/NEWS.d/next/Core and Builtins/2018-07-03-19-00-10.bpo-33418.cfGm3n.rst

+Fix potential memory leak in function object when it creates reference
+cycle.

Objects/funcobject.c

@@ -523,23 +523,31 @@ func_new_impl(PyTypeObject *type, PyCodeObject *code, PyObject *globals,
     return (PyObject *)newfunc;
 }
 
+static int
+func_clear(PyFunctionObject *op)
+{
+    Py_CLEAR(op->func_code);
+    Py_CLEAR(op->func_globals);
+    Py_CLEAR(op->func_module);
+    Py_CLEAR(op->func_name);
+    Py_CLEAR(op->func_defaults);
+    Py_CLEAR(op->func_kwdefaults);
+    Py_CLEAR(op->func_doc);
+    Py_CLEAR(op->func_dict);
+    Py_CLEAR(op->func_closure);
+    Py_CLEAR(op->func_annotations);
+    Py_CLEAR(op->func_qualname);
+    return 0;
+}
+
 static void
 func_dealloc(PyFunctionObject *op)
 {
     _PyObject_GC_UNTRACK(op);
-    if (op->func_weakreflist != NULL)
+    if (op->func_weakreflist != NULL) {
         PyObject_ClearWeakRefs((PyObject *) op);
-    Py_DECREF(op->func_code);
-    Py_DECREF(op->func_globals);
-    Py_XDECREF(op->func_module);
-    Py_DECREF(op->func_name);
-    Py_XDECREF(op->func_defaults);
-    Py_XDECREF(op->func_kwdefaults);
-    Py_XDECREF(op->func_doc);
-    Py_XDECREF(op->func_dict);
-    Py_XDECREF(op->func_closure);
-    Py_XDECREF(op->func_annotations);
-    Py_XDECREF(op->func_qualname);
+    }
+    (void)func_clear(op);
     PyObject_GC_Del(op);
 }
 
@@ -612,7 +620,7 @@ PyTypeObject PyFunction_Type = {
     Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,    /* tp_flags */
     func_new__doc__,                            /* tp_doc */
     (traverseproc)func_traverse,                /* tp_traverse */
-    0,                                          /* tp_clear */
+    (inquiry)func_clear,                        /* tp_clear */
     0,                                          /* tp_richcompare */
     offsetof(PyFunctionObject, func_weakreflist), /* tp_weaklistoffset */
     0,                                          /* tp_iter */