"get_new_mail" fails with "KeyError: 'content-transfer-encoding'" if header is missing

[sumpfralle]

Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/srv/virtualenvs/stadtgestalten/lib/python3.4/site-packages/django/core/management/__init__.py", line 367, in execute_from_command_line
    utility.execute()
  File "/srv/virtualenvs/stadtgestalten/lib/python3.4/site-packages/django/core/management/__init__.py", line 359, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/srv/virtualenvs/stadtgestalten/lib/python3.4/site-packages/django/core/management/base.py", line 294, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/srv/virtualenvs/stadtgestalten/lib/python3.4/site-packages/django/core/management/base.py", line 345, in execute
    output = self.handle(*args, **options)
  File "/srv/virtualenvs/stadtgestalten/lib/python3.4/site-packages/django_mailbox/management/commands/getmail.py", line 24, in handle
    messages = mailbox.get_new_mail()
  File "/srv/virtualenvs/stadtgestalten/lib/python3.4/site-packages/django_mailbox/models.py", line 373, in get_new_mail
    msg = self.process_incoming_message(message)
  File "/srv/virtualenvs/stadtgestalten/lib/python3.4/site-packages/django_mailbox/models.py", line 216, in process_incoming_message
    msg = self._process_message(message)
  File "/srv/virtualenvs/stadtgestalten/lib/python3.4/site-packages/django_mailbox/models.py", line 355, in _process_message
    msg.set_body(message.as_string())
  File "/usr/lib/python3.4/email/message.py", line 159, in as_string
    g.flatten(self, unixfrom=unixfrom)
  File "/usr/lib/python3.4/email/generator.py", line 112, in flatten
    self._write(msg)
  File "/usr/lib/python3.4/email/generator.py", line 186, in _write
    msg.replace_header('content-transfer-encoding', munge_cte[0])
  File "/usr/lib/python3.4/email/message.py", line 559, in replace_header
    raise KeyError(_name)
KeyError: 'content-transfer-encoding'

The root cause seems to be python's issue 27321.