Functionally equivalent, clearer code, English comments.

The changes in mcast.py come from the first version of the patch for issue5379.

many problems with the ancient mcast.py demo code.

true singleton object.

Reviewed by Gregor Lingl.

#687648 from Robert Schuppenies: use classic division.  From me: don't use string exception; add __main__ section

#687648 from Robert Schuppenies: use classic division.  From me: don't use string exception; flush stdout after printing

#687648 from Robert Schuppenies: use classic division.  (RM Barry gave permission to update the demos.)