[3.11] gh-94808: Cover `str.rsplit` for UCS1, UCS2 or UCS4 (GH-98228) (#98291)

gh-94808: Cover `str.rsplit` for UCS1, UCS2 or UCS4 (GH-98228)
(cherry picked from commit b7dd2cad186e44e2b481f4518be62f34c682ea59)

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>