Better treatment of line length limits for the Python inferior

Better treatment of line length limits for the Python inferior

* lisp/comint.el (comint-max-line-length): New constant reflecting a
safe maximum line size that can be sent to an inferior process.
* lisp/progmodes/python.el
(python-shell-comint-watch-for-first-prompt-output-filter): Send setup
code to the inferior process only once and at this stage.
(python-shell-eval-setup-code, python-shell-eval-file-setup-code):
Move, unchanged, to an earlier point to avoid byte-compiler warnings.
(python-shell-send-string-no-output): Revert changes of e32c7d2a8d
(python-shell-send-string): Use 'comint-max-line-length' to decide
when to resort to temp files.
(python-shell-send-string, python-shell-send-file): Don't send setup
code each time (bug#49822).