Fix SafeMIMEText.set_payload() crash on Python 3.13

See django/django@b231bcd

Author: manisandro

flask_mailman/message.py

@@ -159,7 +159,7 @@ def __setitem__(self, name, val):
 
     def set_payload(self, payload, charset=None):
         if charset == 'utf-8' and not isinstance(charset, Charset.Charset):
-            has_long_lines = any(len(line.encode()) > RFC5322_EMAIL_LINE_LENGTH_LIMIT for line in payload.splitlines())
+            has_long_lines = any(len(line.encode(errors="surrogateescape")) > RFC5322_EMAIL_LINE_LENGTH_LIMIT for line in payload.splitlines())
             # Quoted-Printable encoding has the side effect of shortening long
             # lines, if any (#22561).
             charset = utf8_charset_qp if has_long_lines else utf8_charset