rfc822.Message mapping interface

[Postal Meowbot]

According to The Friendly Manual, Message objects from the rfc822
module offer a read-only mapping interface to the headers.  In real
life, __delitem__ and __setitem__ methods exist and even do something
useful (very useful for the application at hand).

In this case, does it mean these methods are internal and may
disappear some day, or just that the docs want updating?

The fate of civiliation is safe in either case.  I'm only trying to
learn if it makes sense to maintain private copies of rfc822 and
mimetools for futureproofing purposes.