wpint: try to shrink the number of customs lines, if needed

python-inema has a method that tries to combine multiple customs
lines into one, if the HTS of those lines is identical.  We forgot
to use it so far.

models/dp_delivery_carrier.py

@@ -191,6 +191,7 @@ class DPDeliveryCarrier(models.Model):
         else:
             wpi_content = []
             total_value = 0
+        wpi_content = wpi.shrink_contents_if_needed(wpi_content)
         wpi_item = wpi.build_item(service.code, wpi_sender, wpi_recipient, weight*1000, total_value,
                                   'EUR', customer_reference=pickings.name, contents=wpi_content)
         # actually create the order + download the label