Since EmailMessage is a provisional API we can fix API bugs in a
maintenance release, but I used a trick suggested by Serhiy to
maintain backward compatibility with 3.4.0/1.
This adds EmailMessage and, MIMEPart subclasses of Message
with new API methods, and a ContentManager class used by
the new methods. Also a new policy setting, content_manager.
Patch was reviewed by Stephen J. Turnbull and Serhiy Storchaka,
and reflects their feedback.
I will ideally add some examples of using the new API to the
documentation before the final release.
R David Murray