mail: document behavior of Python 3 test-notify.t (and possibly other tests) are failing on Python 3 because email.message.Message is now aware of encodings and attempts to roundtrip values with the specified message encoding. Python 2 doesn't perform this roundtripping. We have tests with non-ascii data being serialized to a message that claims to use ascii encoding. I /think/ Mercurial's behavior may be buggy here. But I'm not sure. I'm documenting the behavior so the next person who looks into this doesn't start from scratch like I did. Differential Revision: https://phab.mercurial-scm.org/D5714