use Py_REFCNT(obj) instead of obj->ob_refcnt (allows building in nogil)

[jimkring]

here’s a suggestion from @colesbury:

This looks like it needs to use Py_REFCNT(obj) instead of obj->ob_refcnt. The reference count field changed (it's now two fields in "nogil Python), so you can't access them directly. The Python docs now say to use Py_REFCNT instead of ->ob_refcnt so maybe we can get this change upstreamed.