python: open messages in binary mode

python: open messages in binary mode

currently, notmuch's get_message_parts() opens the file in text mode and passes
the file object to email.message_from_file(fp). In case the email contains
UTF-8 characters, reading might fail inside email.parser with the following exception:

  File "/usr/lib/python3.6/site-packages/notmuch/message.py", line 591, in get_message_parts
    email_msg = email.message_from_binary_file(fp)
  File "/usr/lib/python3.6/email/__init__.py", line 62, in message_from_binary_file
    return BytesParser(*args, **kws).parse(fp)
  File "/usr/lib/python3.6/email/parser.py", line 110, in parse
    return self.parser.parse(fp, headersonly)
  File "/usr/lib/python3.6/email/parser.py", line 54, in parse
    data = fp.read(8192)
  File "/usr/lib/python3.6/codecs.py", line 321, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe4 in position 1865: invalid continuation byte

To fix this, read file in binary mode and pass to
email.message_from_binary_file(fp).

Unfortunately, Python 2 doesn't support
email.message_from_binary_file(fp), so keep using
email.message_from_file(fp) there.

Signed-off-by: Florian Klink <flokli@flokli.de>