(python-mode): Conditionalize imenu initializations to when we can
safely require imenu.  Under Emacs this should prevent python-mode
from hosing the global value of imenu-create-index-function and
messing things up for all other modes.  Problem identified by
Christian Egli.

(py-describe-mode): py-delete-char => py-electric-backspace.  Given by
Christian Egli.