Fix syntax error in [33968]

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@34025 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/mixin/amount_generator.py

@@ -220,7 +220,8 @@ class AmountGeneratorMixin:
             category_list = amount_generator_cell.getCategoryMembershipList(
               amount_generator_cell.getMappedValueBaseCategoryList(), base=1)
             if category_list:
-              property_dict.setdefault('category_list', []) += category_list
+              property_dict.setdefault('category_list',
+                                       []).extend(category_list)
             property_dict['resource'] = resource
             # For final amounts, base_application and id MUST be defined
             property_dict['base_application'] = getBaseApplication() # Required