Define __all__ for rattail.trainwreck.db.model.defaults module

i.e. so custom projects can do this if needed:

    from rattail.trainwreck.db.model.defaults import *

CHANGES.rst

@@ -7,6 +7,8 @@ Unreleased
 
 * Cleanup alembic scripts for trainwreck.
 
+* Define ``__all__`` for ``rattail.trainwreck.db.model.defaults`` module.
+
 
 0.11.12 (2024-04-28)
 --------------------

rattail/trainwreck/db/model/defaults.py

@@ -2,7 +2,7 @@
 ################################################################################
 #
 #  Rattail -- Retail Software Framework
-#  Copyright © 2010-2022 Lance Edgar
+#  Copyright © 2010-2024 Lance Edgar
 #
 #  This file is part of Rattail.
 #
@@ -24,8 +24,6 @@
 Trainwreck *default* data models
 """
 
-from __future__ import unicode_literals, absolute_import
-
 from rattail.trainwreck.db import model
 
 
@@ -33,6 +31,16 @@ from rattail.trainwreck.db import model
 # from rattail.trainwreck.db.model import defaults as trainwreck_model
 Base = model.Base
 
+# nb. this is here for projects which need to do:
+# from rattail.trainwreck.db.model.defaults import *
+__all__ = [
+    'Base',
+    'Transaction',
+    'TransactionOrderMarker',
+    'TransactionItem',
+    'TransactionItemDiscount',
+]
+
 
 class Transaction(model.TransactionBase, model.Base):
     """