L10n:Pontoon/Python 3

Should we support python 3 in Pontoon?

There is an existing bug 1362323.

Pros:

• Django 2.x dropped support for python 2

Cons:

• it's work for untracked gains
• not all dependencies support python 3 yet

Unresolved dependencies:

 django-sslserver
 futures
 mercurial
 pylama
 silme
 test-utils

Work Track:

• https://public.etherpad-mozilla.org/p/pontoon.python3