Respect the delimiter of completer in Python shell completion

Respect the delimiter of completer in Python shell completion

* lisp/progmodes/python.el: (python-shell-completion-setup-code): Fix
the completion code of IPython.  Change the return value to JSON string
and ...
(python-shell-completion-get-completions): ... simplify parsing.
(inferior-python-mode): Update docstring.
(python-shell-readline-completer-delims): New variable indicating the
word delimiters of readline completer.
(python-shell-completion-native-setup): Set the completer delimiter.
(python-shell-completion-native-get-completions): Convert output string
to completions properly.
(python-shell--get-multiline-input)
(python-shell--extra-completion-context)
(python-shell-completion-extra-context): New functions.
(python-shell-completion-at-point): Send text beginning from the line
start if the completion backend does not need word splitting.  Remove
the detection of import statement because it is not needed anymore.
Create proper completion table based on completions returned from
different backends.

* test/lisp/progmodes/python-tests.el (python-tests--completion-module)
(python-tests--completion-parameters)
(python-tests--completion-extra-context): New helper functions.
(python-shell-completion-at-point-jedi-completer)
(python-shell-completion-at-point-ipython): New tests.  (bug#68559)