gh-139103: Improve namedtuple scaling in free-threaded build (gh-144332)

Add `_Py_type_getattro_stackref`, a variant of type attribute lookup
that returns `_PyStackRef` instead of `PyObject*`. This allows returning
deferred references in the free-threaded build, reducing reference count
contention when accessing type attributes.

This significantly improves scaling of namedtuple instantiation across
multiple threads.

* Add blurb

* Rename PyObject_GetAttrStackRef to _PyObject_GetAttrStackRef

* Apply suggestion from @vstinner

Co-authored-by: Victor Stinner <[email protected]>

* Apply suggestion from @vstinner

Co-authored-by: Victor Stinner <[email protected]>

* format

* Update Include/internal/pycore_function.h

Co-authored-by: Victor Stinner <[email protected]>

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Co-authored-by: Victor Stinner <[email protected]>