python.el: Increase native completion robustness.

python.el: Increase native completion robustness.

* lisp/progmodes/python.el (python-shell-completion-native-setup):
Make completer print real candidates and just return dummy ones to
avoid input modification.
(python-shell-completion-native-get-completions): Set
comint-redirect-insert-matching-regexp to non-nil and make
comint-redirect-finished-regexp match the last dummy candidate.
Use python-shell-accept-process-output to wait for the full list
of candidates.