(py-mark-def-or-class): Added an exchange-point-and-mark and a

py-keep-region-active so that the marked def/class gets the
zmacs-region or transient-mark region highlighted.  Also point should
be left at the end of the marked region.

(py-mode-map): Moved py-mark-def-or-class to M-C-h to conform to Emacs
major mode standards.