bpo-38840: Incorrect __all__ in multiprocessing.managers (GH-18034)

This was causing test___all__ to fail on platforms lacking a shared
memory implementation.

Co-Authored-By: Xavier de Gaye <xdegaye@gmail.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>
(cherry picked from commit d0978761118856e8ca8ea7b162a6585b8da83df9)

Co-authored-by: Zackery Spytz <zspytz@gmail.com>