[3.13] gh-132070: Use _PyObject_IsUniquelyReferenced in unicodeobject (gh-133039) (gh-133126)

* gh-132070: Use _PyObject_IsUniquelyReferenced in unicodeobject (gh-133039)

---------
(cherry picked from commit 75cbb8d89e7e92ccaba5c615c72459f241dca8b1)

Co-authored-by: Donghee Na <donghee.na@python.org>
Co-authored-by: Kumar Aditya <kumaraditya@python.org>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

* Add _PyObject_IsUniquelyReferenced

---------

Co-authored-by: Kumar Aditya <kumaraditya@python.org>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>