Disable completion while entering python multiline statements

Disable completion while entering python multiline statements

The "legacy" completion mechanism sends newlines to the running python
process to get the list of completions, which confuses things if the
user is in the middle of entering a multiline statement (Bug#28051).
It's better to disable completion in this case.
* lisp/progmodes/python.el (python-shell--block-prompt): New variable.
(python-shell-prompt-set-calculated-regexps): Set it.
(python-shell-completion-at-point): Return 'ignore' as the completion
function when the current prompt is a block prompt.