L10n:Pontoon/Python 3

We should support python3 in order to upgrade to Django 2.x. We track this in bug 1362323.

Unresolved dependencies:

 django-sslserver: seems to be working well in python3
 pylama: seems to be working well in python3
 silme: we made our for python3 compatible and merged it
 mercurial: we upgraded to version 5.0 with beta python3 support, seems to be working well
 futures: not needed in python3: https://github.com/jazzband/django-pipeline/blob/1.5.3/setup.py#L9