Use duck typing to convert single UPS package into a list

It seems that UPS has fixed its API such that it always returns a list for
PackageResults but it is still better to test the actual result.

Closes #13116

modules/stock_package_shipping_ups/stock.py

@@ -213,8 +213,8 @@ class CreateShippingUPS(Wizard):
         shipment.shipping_reference = (
             shipment_results['ShipmentIdentificationNumber'])
         ups_packages = shipment_results['PackageResults']
-        if len(packages) == 1:
-            # In case only one package is requested UPS returns a dictionnary
+        if not isinstance(ups_packages, list):
+            # In case only one package is requested UPS may return a dictionary
             # instead of a list of one package
             ups_packages = [ups_packages]