Added PortableUnixMailbox to the __all__ variable, and in the __main__ section use this class instead of UnixMailbox as per the comments in the latter's class.
Added PortableUnixMailbox to the __all__ variable, and in the __main__
section use this class instead of UnixMailbox as per the comments in
the latter's class.

Bug fix candidate for 2.2.1.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator