import relation with correct case at module level

__init__.py

@@ -7,7 +7,6 @@
 from .mail_template import MailTemplate  # noqa: F401
 from .model import Model  # noqa: F401
 from .relation import (  # noqa: F401
-    Relation,
     RelationToMany,
     RelationToManyMap,
     RelationToOne,
@@ -11,6 +10,7 @@
     RelationToMany,
     RelationToManyMap,
     RelationToOne,
+    relation,
 )
 from .switch import Switch  # noqa: F401
 from .validate import NotInitialized, Validator  # noqa: F401