(mark-python-def-or-class): Renamed to py-mark-def-or-class globally.

(py-mode-map): Moved py-mark-def-or-class from M-C-h to C-c C-m since
the old binding conflicts with the standard global backward-kill-word
binding, and this new binding is more conformant with other language
modes.  Moved py-mark-block to C-c C-k.