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

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

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

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(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

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