This means that you compiled the extension against a different Python
version than the one you're trying to run it with. (If you use the
mod9.13 script, it uses a built-in version of Python 2.3, so that's
definitely not going to work unless you have a copy of the 2.3 headers
on your system.)
Setting PYTHONPATH will probably just confuse things, since the modpy
script does that already.