[Python-3000] turtle.Screen- how to implement best a Singleton

[Terry Reedy]

> I, for my part, prefer the latter solution (b). Example: a student, 
> having (interactively) produced some design using some turtle t = 
> Turtle() decides spontaneously to change backgroundcolor. s = Screen(); 
> s.bgcolor("pink") should do this for her -

My perhaps naive view is that students should be given name S bound to 
the single Screen object and not have direct access to the Screen class 
object to ever call it.  S.bgcolor should be all that they need to do.