Detect the readline support for Python shell completion

Detect the readline support for Python shell completion

* lisp/progmodes/python.el
(python-shell-comint-watch-for-first-prompt-output-filter):
Detect the readline support.
(python-shell-readline-completer-delims): Update docstring.
(python-shell-completion-native-setup): Move the readline
detection code to ...
(python-shell-readline-detect): ... new function.
(python-shell-completion-native-turn-on-maybe): Skip if Python
has no readline support.
(python-shell-completion-at-point): Respect the delimiter of
readline completer in non-native completion.
* test/lisp/progmodes/python-tests.el
(python-shell-completion-at-point-1)
(python-shell-completion-at-point-native-1)
(python-completion-at-point-1, python-completion-at-point-2)
(python-completion-at-point-pdb-1)
(python-completion-at-point-while-running-1)
(python-completion-at-point-native-1)
(python-completion-at-point-native-2)
(python-completion-at-point-native-with-ffap-1)
(python-completion-at-point-native-with-eldoc-1): Skip tests if
Python has no readline support.
(python-shell-completion-at-point-jedi-completer): Add test for
non-native Python shell completion. (bug#68559)