bpo-43770: Refactor type_new() function (GH-25325)

* Split type_new() into into many small functions.
* Add type_new_ctx structure to pass variables between subfunctions.
* Initialize some PyTypeObject and PyHeapTypeObject members earlier
  in type_new_alloc().
* Rename variables to more specific names.
* Add "__weakref__" identifier for type_new_visit_slots().
* Factorize code to convert a method to a classmethod
  (__init_subclass__ and __class_getitem__).
* Add braces to respect PEP 7.
* Move variable declarations where the variables are initialized.

Objects/dictobject.c

@@ -4972,10 +4972,12 @@ PyDictKeysObject *
 _PyDict_NewKeysForClass(void)
 {
     PyDictKeysObject *keys = new_keys_object(PyDict_MINSIZE);
-    if (keys == NULL)
+    if (keys == NULL) {
         PyErr_Clear();
-    else
+    }
+    else {
         keys->dk_lookup = lookdict_split;
+    }
     return keys;
 }