Thanks for finding this. The bug is in my (new) test for
mailer.standard_message not in the things it tests.

The roundup mailer interface in python3 uses python strings (unicode)
while python2 uses utf-8. So we get a traceback in python3 when it tries
to *encode* a utf-8 bytes type. The fix is to have the test distinguish
these two cases, looks ugly but I don't think we can avoid the
dependency on the python versions because of the different APIs.

Sorry for not checking if my test passed for py3, I'll try to do that in
the future.

Fix pushed in f767a83a6af0