(py-backspace-function): New variable.

(py-electric-backspace, py-electric-delete): Support the XEmacs 20 Way
for backspace and delete mappings.  In XEmacs 19, Emacs 19, and Emacs
20, both backspace and delete keysyms are bound to
py-electric-backspace.  In XEmacs 20, backspace and delete keysyms are
bound separately, allowing the user to specify forward or backward
deletion of the delete keysym through the variable
delete-key-deletes-forward.  All this is the Right Way To Do It and
this implementation was largely ripped from CC Mode.