Implement caching for 'python-shell-completion-at-point'

Implement caching for 'python-shell-completion-at-point'

* lisp/progmodes/python.el (python-shell-completion-at-point): cache
results, since computing them involves talking with the inferior
process and, potentially, network communications
(python-shell--capf-cache): new variable, for cache
(python-shell-completion-get-completions)
(python-shell-completion-native-get-completions): 'import' argument is
not needed anymore.
(python-shell-completion-native-setup)
(python-shell-completion-native-try): pass the setup code
synchronously, to avoid printing a message in the shell (bug#50459).