Use relative imports

[htgoebel]

https://github.com/coopengo/tryton-training/wiki/5.0-step1 reads:

Open the init.py file and write down the following:
…
import library

For clarity and expressiveness, this should become a relative import. Current officail tryton modules use relatvice imports, too, see e.g. https://hg.tryton.org/modules/commission/file/tip/__init__.py#l5

Anyhow, I wonder how this does work at all, as in Python 3 this is an absolute import and will import a completly different module.

[notzippy]

Duplicate of #8