T390-python: add test for get_message_parts and special characters

This imports a message with ISO-8859-2 encoded characters, then opens
the database using the python bindings. We peek through all mesage
parts, afterwards print the message id.

Signed-off-by: Florian Klink <flokli@flokli.de>
Signed-off-by: Andreas Rammhold <andreas@rammhold.de>