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

[Mark Dickinson]

Mark Dickinson added the comment:

I just checked in another very minor change in r60723.  The repr of a 
Fraction now looks like Fraction(1, 2) instead of Fraction(1,2).  Let me 
know if this change is more controversial than I think it is.

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