[3.13] gh-126476: Raise IllegalMonthError for calendar.formatmonth() when the input month is not correct (GH-126484) (GH-126879)

gh-126476: Raise IllegalMonthError for calendar.formatmonth() when the input month is not correct (GH-126484)
(cherry picked from commit 3be7498d2450519d5d8f63a35ef298db3b3d935b)

Co-authored-by: Nadeshiko Manju <me@manjusaka.me>
Co-authored-by: Ethan Furman <ethan@stoneleaf.us>