email2trac: use EmailMessage for email parsing.
Parse messages using newer python3.6 API email.message.EmailMessage instead of old email.message.Message.
Adapt further message processing to new API, where everything is a Unicode string.
All available email2trac message parsing options were tested (for creating a new Trac ticket).
All modern message encodings and charsets were tested.
Also tested with various messages from: https://github.com/apache/spamassassin/tree/trunk/t/data
Fixes most (all?) email parsing errors with python3 (#1)