Old python syntax for 5.0

[pierre-malis-coopengo]

Some of your commits have file with old python syntax:

import library

new python syntax must be :

from . import library

. being the current directory, might be a sub directory if you like.

Minor problem but affects a lot (or all) of your branches,
here is a few of them:

https://github.com/coopengo/tryton-training/blob/5.0/step10_completed/modules/library/__init__.py

https://github.com/coopengo/tryton-training/blob/5.0/step7_homework/modules/library/__init__.py

https://github.com/coopengo/tryton-training/blob/5.0/step4_completed/modules/library/__init__.py