[3.11] bpo-36959: Fix error messages for invalid ISO format string in _strptime() (GH-13408) (GH-113499)

Previously some error messages complained about incompatible
combinations of directives that are not contained in the format string.

(cherry picked from commit 4b2c3e8e436b5191039cbe8cd9932654a60803e6)

Co-authored-by: Gordon P. Hemsley <me@gphemsley.org>