Introductory comment updates.

(python-font-lock-keywords): Added "assert"

(py-block-closing-keywords-re): New variable.

(py-no-outdent-re): Rewrite to use py-block-closing-keywords-re.

(py-shell): py-process-filter should no longer be necessary.  Comint
should do all the work.  Note that more fixes to the py-shell process
mechanism need to be done.

(py-execute-region): Check for empty region.  Some questionable
changes to set-buffer after shell-command-on-region.  Again, this all
needs to be closely examined for X/Emacs 19/20 compatibility.

(py-goto-beyond-final-line): py-parse-partial-sexp-works-p should no
longer be necessary.

(py-statement-closes-block-p): Use py-block-closing-keywords-re.