More robust shell startup and code setup.

More robust shell startup and code setup.

* lisp/progmodes/python.el (python-shell-make-comint): Remove
accept-process-output call.
(python-shell-get-buffer): Return current buffer if major-mode is
inferior-python-mode.
(python-shell-get-or-create-process): Use it.
(python-shell-send-setup-code): Send all setup code in one string,
output success message and accept-process-output.