Unify po file support
In tryton, we use Babel to extract po file and compile them into mo file.
In trytond, we use polib to parse and create po file.
polib has fewer users [1] and did not have a release since 2 years (almost the same for commits).
Babel has support for po file [2] but also have support for pluralization which may be used in the future.
I think we should use Babel for both.
[1] https://libraries.io/pypi/polib
[2] http://babel.pocoo.org/en/latest/api/messages/pofile.html