When we updated rbintegrations to work with Review Board 5.0, we only

really did the minimum required to make things work. Because

rbintegrations 3.0+ will only be used with Review Board 5.0+ / Django

3.2+ / Python 3.7+, we can do a lot more. This change updates the

codebase to handle some additional modernization tasks:

Remove __future__ imports.
Switch from django.conf.urls to django.urls and swap out url() for

  the new path().
Move from ugettext to gettext.
Eliminate fallback imports and conditional logic for handling various

  versions of Review Board.

Ran unit tests.
Did some superficial testing setting up various integration

  configurations.