[3.12] gh-77749: Fix inconsistent behavior of non-ASCII handling in EmailPolicy.fold() (GH-6986) (GH-114606)

It now always encodes non-ASCII characters in headers if utf8 is false.

(cherry picked from commit 504334c7be5a56237df2598d338cd494a42fca4c)

Co-authored-by: Rito Takeuchi <licht-t@outlook.jp>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>