python.el: Fix completion for pdb interactions

python.el: Fix completion for pdb interactions

* lisp/progmodes/python.el (python-shell-completion-setup-code):
Simplify.  Toggle print_mode for native wrapped completer.
(python-shell-completion-native-setup): Ensure process buffer.
Add print_mode attribute to completer wrapper to toggle returning
or printing candidates.
(python-shell-completion-native-get-completions): Cleanup.
(python-shell-completion-get-completions): Cleanup.
(python-shell-completion-at-point): Perform prompt checks.  Force
fallback completion in pdb interactions.