[3.13] gh-131434: Improve error reporting for incorrect format in strptime() (GH-131568) (GH-132309)

In particularly, fix regression in detecting stray % at the end of the
format string.
(cherry picked from commit 3feac7a093b3fcd549c5dc54277f26f585f2ab0c)

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