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. Testing Done: - Ran unit tests. - Did some superficial testing setting up various integration configurations.