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.