Get rid of borrowed global singletons.

Summary:
Add a PyAPI declaration to the objects we need and remove the borrows from them
and their dependencies.

Also remove the initialisations in `init_upstream_borrow()`; this is now an
empty function retained to avoid `#ifdef`ing it out in cinder code.

Reviewed By: itamaro

Differential Revision: D65372443

fbshipit-source-id: a216a6c75807bf1abe55eb068ebba0523b934542

Include/internal/pycore_unionobject.h

@@ -8,7 +8,7 @@ extern "C" {
 #  error "this header requires Py_BUILD_CORE define"
 #endif
 
-extern PyTypeObject _PyUnion_Type;
+PyAPI_DATA(PyTypeObject) _PyUnion_Type;
 #define _PyUnion_Check(op) Py_IS_TYPE((op), &_PyUnion_Type)
 PyAPI_FUNC(PyObject *) _Py_union_type_or(PyObject *, PyObject *);