Refs #34900 -- Fixed SafeMIMEText.set_payload() crash on Python 3.13.

Payloads with surrogates are passed to the set_payload() since
python/cpython@f97f25e

django/core/mail/message.py

@@ -168,7 +168,8 @@ 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
+                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