menu "Python/Start interpreter" offers choice

"Default Python Shell" resp. "Alternative Interpreter"
delete comment forms added
py-empty-out-list-backward, new command

Deletes all elements from list before point
`py-execute-statement' fixed
menu Python/py-execute-... reconsidered
menu Python/Interpreter-... reconsidered
syntax-table stuff cleanup
enhance XEmacs compatibility, lp:1177156
nested-dictionaries-indent-again, lp:1174174, fixed