Support POS sale line columns defined before mixin is applied

The mixin is applied after the Model so if another module wants to extend the
method, the method may be in the super class.

modules/sale_point/sale_reporting.py

@@ -37,6 +37,11 @@ class AbstractMixin:
 
     @classmethod
     def _pos_sale_line_columns(cls, line, point, sale, currency):
+        try:
+            columns = super()._pos_sale_line_columns(
+                line, point, sale, currency)
+        except AttributeError:
+            columns = []
         return [
             line.product.as_('product'),
             line.quantity.as_('quantity'),
@@ -48,7 +53,7 @@ class AbstractMixin:
             Literal(None).as_('customer'),
             point.storage_location.as_('location'),
             point.address.as_('shipment_address'),
-            ]
+            ] + columns
 
     @classmethod
     def _lines(cls):