The following fixes the example:
from copy import copy
from io import BytesIO
from email.message import Message
from email.generator import BytesGenerator, _has_surrogates
from email._policybase import Compat32


class FixedBytesGenerator(BytesGenerator):
    def _handle_text(self, msg):
        payload = msg._payload
        if payload is None:
            return
        charset = msg.get_param("charset")
        if charset is not None \
               and not self.policy.cte_type=='7bit' \
               and not _has_surrogates(payload):
            msg = copy(msg)
            msg._payload = payload.encode(charset).decode(
                "ascii", "surrogateescape")
        super()._handle_text(msg)
                
    _writeBody = _handle_text


class FixedMessage(Message):
    def as_bytes(self, unixfrom=False, policy=None):
        policy = self.policy if policy is None else policy
        fp = BytesIO()
        g = FixedBytesGenerator(fp, mangle_from_=False, policy=policy)
        g.flatten(self, unixfrom=unixfrom)
        return fp.getvalue()
        

fixed_policy = Compat32(message_factory=FixedMessage)

ms = message_from_string(mt, policy=fixed_policy)
ms.as_bytes()