bpo-43323: Fix UnicodeEncodeError in the email module (GH-32137)

It was raised if the charset itself contains characters not encodable
in UTF-8 (in particular \udcxx characters representing non-decodable
bytes in the source).
(cherry picked from commit e91dee87edcf6dee5dd78053004d76e5f05456d4)

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