Updates to Building Skills in Python

I got a bug report (back in April) about an exercise in Building Skills in Python.  It was a change from 2.2 that I never validated in 2.5.   Thanks to my readers for responding with questions and complaints.

I've finally updated and posted the revisions.

Further, after some questions on Stack Overflow, I've decided to revisit parts of Chapter 21.  Specifically this question leads me to conclude that there's an audience that's served by a little more depth in this area.