migrations: Import BaseDatabaseSchemaEditor from its canonical module.

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

zerver/migrations/0057_realmauditlog.py

@@ -2,7 +2,7 @@
 import django.db.models.deletion
 from django.conf import settings
 from django.db import migrations, models
-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.utils.timezone import now as timezone_now