parser tests: test parsing of interesting mail

parser tests: test parsing of interesting mail

We have 4 new tests:
 - One with a header with a character that is neither 7-bit ASCII
   nor UTF-8.
 - One with UTF-8 in the body.
 - One with un-encoded UTF-8 in the headers.
 - One with a header that claims to be UTF-8 but isn't.

This also requires us to change how we open files on Python 3. Use
the binary file and message_from_binary_file setup under Python 3.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephen@that.guru>