fix OwnPermissionsMixin save_model method

contracts/admin.py

@@ -71,13 +71,13 @@ def permissions_mixin_factory(
 get_obj_contract = lambda obj: obj.contract
 
 
-OwnPermissionsMixin = permissions_mixin_factory(
+class OwnPermissionsMixin(
+    permissions_mixin_factory(
         "contracts.edit_others",
         "contracts.delete_others",
         lambda request, obj: obj.created_by != request.user,
     )
-
-
+):
     def own_permissions_mixin_save_model(self, request, obj, form, change):
         if obj.created_by is None:
             obj.created_by = request.user
@@ -85,9 +85,6 @@ def own_permissions_mixin_save_model(self, request, obj, form, change):
         return super().save_model(request, obj, form, change)
 
 
-OwnPermissionsMixin.save_model = own_permissions_mixin_save_model
-
-
 ParentContractApprovedPermissionsMixin = permissions_mixin_factory(
     "contracts.edit_when_approved",
     "contracts.delete_when_approved",