How do I get a mimetools.Message object from a mailbox.UnixMailbox object?

[Paul Moore]

I'm parsing a Unix mbox format file in Python. No problem, that's what
the mailbox module does...

The problem is, mbx.next() returns a rfc822.Message instance - and my
mailbox contains MIME-encoded messages with attachments. How do I get
from the rfc822.Message instance to a mimetools.Message instance?

Thanks,
Paul Moore