el/dot-emacs.el (mdw-fontify-pythonic): Handle word boundaries better.

For some reason, in squeeze's Emacs, underscores cause a break in the
middle of a token, and the individual pieces can be recognized as
keywords or numbers.  This is obviously wrong.  Fix it by using
different word-boundary regexp magic.

This doesn't seem to happen in wheezy's Emacs, but I don't think this
change will harm that.