The rejection of PEP 239 was years ago.  PEP 3141 was accepted; it
includes adding a rational type to the stdlib, and Jeffrey is doing this
with my encouragement.

The motivation is to have at least one example of each type in our
modest numeric tower in the stdlib.  I'm sure there will be plenty of
uses for rational.py, e.g. in education.  Of course, if someone has a
better candidate or a proposed improvement, speak up now!  It looks like
Mark's suggestions can be treated as code review; I don't think we need
a new PEP.

Note that the mere existence of PEP 239 again points out that the demand
is not zero.