[3.12] gh-80361: Fix TypeError in email.Message.get_payload() (GH-117994) (GH-117998)

It was raised when the charset is rfc2231 encoded, e.g.:

   Content-Type: text/plain; charset*=ansi-x3.4-1968''utf-8
(cherry picked from commit deaecb88fa5da68cbffca413c63af95fd99578dd)

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