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>

pgroonga/migrations/0001_enable.py

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

pgroonga/migrations/0002_html_escape_subject.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 from django.db import connection, migrations
 from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
 from django.db.migrations.state import StateApps

pgroonga/migrations/0003_v2_api_upgrade.py

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