Extend grouping fields with a list

4e84027dbc2e updated the grouping fields method to receive the grouped
instance but it did not adapt to the returned type which is a list.

Closes #12718

modules/incoterm/sale.py

@@ -32,8 +32,8 @@ class Sale(IncotermAvailableMixin, metaclass=PoolMeta):
         return shipment
 
     def _get_shipment_grouping_fields(self, shipment):
-        return super()._get_shipment_grouping_fields + (
-            'incoterm', 'incoterm_location')
+        return super()._get_shipment_grouping_fields(shipment) + [
+            'incoterm', 'incoterm_location']
 
     @property
     @fields.depends('company', 'warehouse', 'shipment_address')

modules/sale_invoice_date/sale.py

@@ -129,5 +129,5 @@ class Sale(metaclass=PoolMeta):
         return invoice
 
     def _get_invoice_grouping_fields(self, invoice):
-        return super()._get_invoice_grouping_fields(invoice) + (
-            'invoice_date',)
+        return super()._get_invoice_grouping_fields(invoice) + [
+            'invoice_date']

modules/sale_shipment_cost/sale.py

@@ -357,8 +357,8 @@ class Sale(metaclass=PoolMeta):
         return cost_line
 
     def _get_shipment_grouping_fields(self, shipment):
-        return super()._get_shipment_grouping_fields(shipment) + (
-            'cost_method', 'carrier')
+        return super()._get_shipment_grouping_fields(shipment) + [
+            'cost_method', 'carrier']
 
     @property
     def shipment_cost_amount(self):