C extension fails to compile on free-threaded Python 3.14t

[jensens]

`_pickle_33.c` uses direct struct member access that is incompatible with free-threaded Python 3.14t (PEP 703):

• Line 720: `(self)->arg->ob_refcnt > 1` — direct `ob_refcnt` access fails because free-threaded Python uses atomic refcounting
• Line 1528: `obj->ob_type->tp_name` — direct `ob_type` access fails because free-threaded Python uses atomic type lookups

Both should use the CPython API macros (`Py_REFCNT()`, `Py_TYPE()`) which handle both standard and free-threaded builds.