bpo-40170: PyObject_NEW() becomes an alias to PyObject_New() (GH-19379)

The PyObject_NEW() macro becomes an alias to the PyObject_New()
macro, and the PyObject_NEW_VAR() macro becomes an alias to the
PyObject_NewVar() macro, to hide implementation details. They no
longer access directly the PyTypeObject.tp_basicsize member.

Exclude _PyObject_SIZE() and _PyObject_VAR_SIZE() macros from
the limited C API.

Replace PyObject_NEW() with PyObject_New() and replace
PyObject_NEW_VAR() with PyObject_NewVar().