[3.13] gh-140911: Ensure that UserString.index() and UserString.rindex() accept UserString as argument (GH-140945) (GH-141945)

(cherry picked from commit e6174ee9812f514b42ab6d3a1064bd6057de96c7)

Co-authored-by: Krishna Chaitanya <141550576+XChaitanyaX@users.noreply.github.com>