python: Modernize legacy Python 2 syntax with pyupgrade.

Generated by `pyupgrade --py3-plus --keep-percent-format` on all our
Python code except `zthumbor` and `zulip-ec2-configure-interfaces`,
followed by manual indentation fixes.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

zerver/migrations/0113_default_stream_group.py

@@ -1,6 +1,4 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.11.5 on 2017-10-14 15:12
-from __future__ import unicode_literals
 
 import django.db.models.deletion
 from django.db import migrations, models
@@ -24,6 +22,6 @@ class Migration(migrations.Migration):
         ),
         migrations.AlterUniqueTogether(
             name='defaultstreamgroup',
-            unique_together=set([('realm', 'name')]),
+            unique_together={('realm', 'name')},
         ),
     ]