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>

confirmation/migrations/0001_initial.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 from django.db import models, migrations
 import django.db.models.deletion
 

confirmation/migrations/0002_realmcreationkey.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 from django.db import models, migrations
 import django.utils.timezone
 

confirmation/migrations/0003_emailchangeconfirmation.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.10.4 on 2017-01-17 09:16
 from django.db import migrations
 

confirmation/migrations/0004_remove_confirmationmanager.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.11.2 on 2017-07-08 04:23
 from django.db import migrations, models
 

confirmation/migrations/0005_confirmation_realm.py

@@ -1,6 +1,4 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.11.6 on 2017-11-30 00:13
-from __future__ import unicode_literals
 
 from django.db import migrations, models
 import django.db.models.deletion

confirmation/migrations/0006_realmcreationkey_presume_email_valid.py

@@ -1,6 +1,4 @@
-# -*- coding: utf-8 -*-
 # Generated by Django 1.11.6 on 2018-01-29 18:39
-from __future__ import unicode_literals
 
 from django.db import migrations, models