python.el: Defer shell setup code until first interactive prompt

python.el: Defer shell setup code until first interactive prompt

* lisp/progmodes/python.el
(python-shell-comint-watch-for-first-prompt-output-filter): New
function.
(inferior-python-mode): Use it.
(python-shell-first-prompt-hook): New hook.
(python-shell-send-setup-code)
(python-shell-completion-native-turn-on-maybe-with-msg): Attach to
this hook instead of inferior-python-hook.