docs/manual: PEP8 coding style for Python scripts

docs/manual: PEP8 coding style for Python scripts

The advantages of using a pre-existing coding style instead of creating
our own are:
- documenting on the manual takes a single sentence;
- there are automatic tools to help during development/review.

So document that PEP8 recommendation should be followed.

Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>