py3: Remove all from __future__ import unicode_literals.

This was mostly used in migrations, so it's a pretty safe change.

zilencer/migrations/0002_remote_zulip_server.py

@@ -1,6 +1,4 @@
 # -*- coding: utf-8 -*-
-from __future__ import unicode_literals
-
 from django.db import migrations, models
 import django.db.models.deletion