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

[Guido van Rossum]

Guido van Rossum added the comment:

FWIW, if Rational(Decimal(...)) is to be accepted, then
Decimal(Rational(...)) should also be accepted, and arguably mixed
binary operations as well (Rational(...) + Decimal(...) etc.).

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assignee: gvanrossum -> jyasskin

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<http://bugs.python.org/issue1682>
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