UnicodeDecodeError when handling messages containing £

[matburnham]

Sending an email containing a £ symbol raises a UnicodeDecodeError:

======== Running on http://127.0.0.1:1080 ========
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Message from oscar@example.com
SMTP session exception
Traceback (most recent call last):
  File "/home/mat/.virtualenvs/mysite/lib/python3.6/site-packages/aiosmtpd/smtp.py", line 315, in _handle_client
    await method(arg)
  File "/home/mat/.virtualenvs/mysite/lib/python3.6/site-packages/aiosmtpd/smtp.py", line 690, in smtp_DATA
    status = await self._call_handler_hook('DATA')
  File "/home/mat/.virtualenvs/mysite/lib/python3.6/site-packages/aiosmtpd/smtp.py", line 122, in _call_handler_hook
    status = await hook(self, self.session, self.envelope, *args)
  File "/home/mat/.virtualenvs/mysite/lib/python3.6/site-packages/mailmitt/smtp_server.py", line 47, in handle_DATA
    for part in message.walk()
  File "/home/mat/.virtualenvs/mysite/lib/python3.6/site-packages/mailmitt/smtp_server.py", line 48, in <listcomp>
    if not part.is_multipart()
  File "/home/mat/.virtualenvs/mysite/lib/python3.6/site-packages/mailmitt/smtp_server.py", line 25, in extract_part
    "body": part.get_payload(decode=True).decode(charset),
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xa3 in position 174: invalid start byte