python.el: fallback completion, ffap and eldoc setup enhancements

python.el: fallback completion, ffap and eldoc setup enhancements

Setup codes are now sent continuously so that the current frame is
always taken into account.  This allows working within debuggers
and always keeping a fresh version of setup codes that will return
proper results.

* lisp/progmodes/python.el (python-shell-setup-codes): Cleanup.
(python-shell-send-setup-code): Send code only when
python-shell-setup-codes is non-nil.
(python-shell-completion-string-code): Cleanup trailing newline.
(python-shell-completion-get-completions): Always use
python-shell-completion-setup-code.
(python-ffap-setup-code): Work with any object, not only modules.
(python-ffap-string-code): Cleanup trailing newline.
(python-ffap-module-path): Always use python-ffap-setup-code.
(python-eldoc-string-code): Cleanup trailing newline.
(python-eldoc--get-doc-at-point): Always use
python-eldoc-setup-code.  Return non-nil only if docstring is
found.