migrations: Use django.db.backends.postgresql.schema.

This replaces django.db.backends.postgresql_psycopg2, which has been
an alias to django.db.backends.postgresql since Django 1.9.

zerver/migrations/0209_user_profile_no_empty_password.py

@@ -6,7 +6,7 @@ import ujson
 from django.conf import settings
 from django.contrib.auth.hashers import check_password, make_password
 from django.db import migrations
-from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
+from django.db.backends.postgresql.schema import DatabaseSchemaEditor
 from django.db.migrations.state import StateApps
 from django.utils.timezone import now as timezone_now