Which Python version to code for?

[Charles Crain]

I am part of the team working on SCons, the build tool for the Software
Carpentry project.  We have an interesting quandry before us: what is the
earliest version of Python we should support?  On the one hand, we would
like to use the latest and greatest and be able to take advantage of the
several powerful features added in recent Python releases.  On the other
hand, we would like to avoid putting any barriers to adoption of our product
that might arise from requiring users (particularly those in large
organizations with high-intertia IT departments) to upgrade Python versions
in order to use it.

It occurs to us that this is a general question facing anyone wishing to
release a Python product.  Do you code for the latest and greatest, or do
you try to figure out what release is most "common" (in this case, still
1.5.2, since it ships with most major Linux distros) and code for that?  Is
there any consensus as to what to do here?

In particular, does anyone have a feeling for how pmuch of a pain people
consider upgrading Python versions so that they may run a particular piece
of software?  In my experience, upgrading Python versions, and even running
concurrent versions, is pretty easy.

Thanks for anyone that can help out.

-Charles Crain