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/0206_stream_rendered_description.py

@@ -1,5 +1,5 @@
 from django.db import migrations, models
-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 zerver.lib.actions import render_stream_description