Parse completion input in a iPython friendly way.

Parse completion input in a iPython friendly way.

* lisp/progmodes/python.el
(python-shell-completion-at-point): Rename from
python-shell-completion-complete-at-point.
(inferior-python-mode): Use it.
(python-completion-at-point): Rename from
python-completion-complete-at-point.  Parse input up to first
backward occurrence of whitespace, open-paren, close-paren or
string delimiter.
(python-mode): Use it.

Fixes: debbugs:18084