Running source code

py-execute-... commands may use backends different from default according to options

* python-mode-v5-behavior-p
* py-execute-no-temp-p
* py-fast-process-p

python-mode.el v5 series used
`shell-command-on-region'. At that time existed bug
lp:550661, WRT unicode symbol u'\xA9', ©

That bug was avoided by running the stuff from a
temporary file. Which is the default still. An
error-message sent from Python than points to the
temporary file - which requires to be dealt with at the
Emacs side.

As a user wanted to stay with simpler old proceeding,
 `python-mode-v5-behavior-p' was provided as an option.

In order to speed up and/or execute on systems without
write-permission, a newer backend avoids writing
temporary files. Internally relies on
`process-send-string'. Customize `py-execute-no-temp-p'
to enable it.

With v6 series stuff is processed by an interactive
(I)Python shell. That allows further interactive
investigations. The backside was shown by lp:1253907 -
with large output, the comint bookkeeping might make
Emacs slow, resp. freeze.

Thus option `py-fast-process-p' was introduced - see
menu "Python/Fast process..." Avoids Python shell,
results arrive in buffer "*Python Output*" by default.

With version 6.1.3, optional `py-fast-process-p' is not
released yet, available in trunk.