[issue1682] Move Demo/classes/Rat.py to Lib/rational.py and fix it up.

[Mark Dickinson]

Mark Dickinson added the comment:

Guido:

> Correct -- the thing is that you can't know the signature of the
> subclass's constructor so you can't very well blindly call that.

Jeffrey, is it okay for me to change the three class methods 
(from_float, from_decimal, from_continued_fraction) to static methods, 
and call Rational instead of cls as the constructor?

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