Make config.py Python3 compatible
For #3211 (closed), the API of configparser getter is changed for Python3 in such way that it is not possible to make the Tryton override strictly compatible.
Mainly because internally configparse reuse the get method for all the others so the specific signature of Tryton make it not workable.
My solution is to change the signature of the getter to be more flexible using *args, **kwargs. It is for me the best compromise to get support of both versions.
The drawback is that the default value should always be passed using keywords (which is not so bad, explicit is better than implicit)