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

[Mark Dickinson]

Mark Dickinson added the comment:

Okay, Nick; you've got me convinced.  For some reason I wasn't really 
thinking of these methods as alternative constructors---in this light, 
your arguments about doing the same as __new__, and about established 
patterns, make perfect sense.

Looking back at the discussion, Jeffrey looked like he was +/-0 on the 
change, and Guido was answering a slightly different question (about 
__add__ instead of constructors);  I then took his answer out of context 
to justify the change  :(

So I'll change this back unless there's further discussion.

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