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

[Mark Dickinson]

Mark Dickinson added the comment:

staticmethod substituted for classmethod in r60712.

I'm not sure I like the idea of names Rational and Fraction;  the two 
classes numbers.Rational and rational.Rational are quite different beasts, 
and using two almost-synonyms for their names seems like a bad idea.
Is there some more descriptive name for numbers.Rational that might give a 
hint of its ABC-ness?

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