Make 'python -tt' the default, meaning Python won't allow mixing tabs and

Make 'python -tt' the default, meaning Python won't allow mixing tabs and
spaces for indentation. Adds a '-ttt' option to turn the errors back into
warnings; I'm not yet sure whether that's desireable for Py3K.

Also remove the magic for setting the size of tabs based on emacs/vim-style
comments. Python now always considers tabstops to be every-8-spaces.