python.el: Native readline completion.

python.el: Native readline completion.

This commit adds native readline completion that fallbacks to the old
mechanism when it cannot be used for the current interpreter.

* lisp/progmodes/python.el (python-shell-completion-native-disabled-interpreters)
(python-shell-completion-native-enable)
(python-shell-completion-native-output-timeout): New defcustoms.
(python-shell-completion-native-interpreter-disabled-p)
(python-shell-completion-native-try)
(python-shell-completion-native-setup)
(python-shell-completion-native-turn-off)
(python-shell-completion-native-turn-on)
(python-shell-completion-native-turn-on-maybe)
(python-shell-completion-native-turn-on-maybe-with-msg)
(python-shell-completion-native-toggle): New functions.
(python-shell-completion-native-get-completions): New function.
(python-shell-completion-at-point): Use it.

* test/automated/python-tests.el
(python-shell-completion-native-interpreter-disabled-p-1): New
test.