* bpo-33972: Fix EmailMessage.iter_attachments raising AttributeError.
When certain malformed messages have content-type set to 'mulitpart/*' but
still have a single part body, iter_attachments can raise AttributeError. This
patch fixes it by returning a None value instead when the body is single part.
It turns out we can't depend on email.message getting imported every place
message_factory is needed, so to avoid a circular import we need to special
case Policy.message_factory=None in the parser instead of using monkey
patching. I had a feeling that was a bad idea when I did it.
This is a wholesale reorganization and editing of the email documentation to
make the new API the standard one, and the old API the 'legacy' one. The
default is still the compat32 policy, for backward compatibility. We will
change that eventually.
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 is a backward compatible partial fix, the complete fix requires raising
an error instead of accepting the invalid input, so the real fix is only
suitable for 3.4.
