migrations: Import BaseDatabaseSchemaEditor from its canonical module.

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

zerver/migrations/0284_convert_realm_admins_to_realm_owners.py

@@ -3,7 +3,7 @@ from typing import Any, Dict
 
 import orjson
 from django.db import migrations
-from django.db.backends.postgresql.schema import BaseDatabaseSchemaEditor
+from django.db.backends.base.schema import BaseDatabaseSchemaEditor
 from django.db.migrations.state import StateApps
 from django.db.models import Count
 from django.utils.timezone import now as timezone_now