[3.12] gh-113594: Fix UnicodeEncodeError in TokenList.fold() (GH-113730) (GH-113907)

It occurred when try to re-encode an unknown-8bit part combined with non-unknown-8bit part.
(cherry picked from commit e9d5b6ea2d68564f176fdf70c2d7028e060c62b5)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>