email with multipart/related content does not round-trip parsebytes/as_bytes

Bug report

Bug description:

import email
import email.parser
import email.policy

with open('sample_email.eml', 'rb') as f:
    raw_email = f.read()

policy = email.policy.default.clone(cte_type='8bit', utf8=True, refold_source='none')
parser = email.parser.BytesParser(policy=policy)
decoded_email = parser.parsebytes(raw_email)
print("Defects: %s" % decoded_email.defects)
reencoded_email = decoded_email.as_bytes(policy=policy)
print("Two messages matches: %s" % (raw_email == reencoded_email))

sample_email.txt (save it as sample_email.eml)

When ran on different python versions (including 3.13), it raises UnicodeEncodeError:

Defects: []
Traceback (most recent call last):
  File "/var/tmp/etienne/sample_email.py", line 13, in <module>
    reencoded_email = decoded_email.as_bytes(policy=policy)
  File "/usr/lib/python3.13/email/message.py", line 208, in as_bytes
    g.flatten(self, unixfrom=unixfrom)
    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/email/generator.py", line 117, in flatten
    self._write(msg)
    ~~~~~~~~~~~^^^^^
  File "/usr/lib/python3.13/email/generator.py", line 182, in _write
    self._dispatch(msg)
    ~~~~~~~~~~~~~~^^^^^
  File "/usr/lib/python3.13/email/generator.py", line 219, in _dispatch
    meth(msg)
    ~~~~^^^^^
  File "/usr/lib/python3.13/email/generator.py", line 286, in _handle_multipart
    g.flatten(part, unixfrom=False, linesep=self._NL)
    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/email/generator.py", line 117, in flatten
    self._write(msg)
    ~~~~~~~~~~~^^^^^
  File "/usr/lib/python3.13/email/generator.py", line 182, in _write
    self._dispatch(msg)
    ~~~~~~~~~~~~~~^^^^^
  File "/usr/lib/python3.13/email/generator.py", line 219, in _dispatch
    meth(msg)
    ~~~~^^^^^
  File "/usr/lib/python3.13/email/generator.py", line 278, in _handle_multipart
    self.write(subparts)
    ~~~~~~~~~~^^^^^^^^^^
  File "/usr/lib/python3.13/email/generator.py", line 420, in write
    self._fp.write(s.encode('ascii', 'surrogateescape'))
                   ~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
UnicodeEncodeError: 'ascii' codec can't encode characters in position 330-333: ordinal not in range(128)

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-136848

Reproduce the example with email.parser.BytesParser and the supplied sample_email.eml, then trace email/message.py into email/generator.py, especially _handle_multipart and write. Done means as_bytes(policy=policy) completes without UnicodeEncodeError and the reencoded bytes match the original input; review linked PR gh-136848 before starting.