subclasses.

MIMENonMultipart: Base class for non-multipart/* content type subclass
specializations, e.g. image/gif.  This class overrides attach() which
raises an exception, since it makes no sense to attach a subpart to
e.g. an image/gif message.

MIMEMultipart: Base class for multipart/* content type subclass
specializations, e.g. multipart/mixed.  Does little more than provide
a useful constructor.