Fix returning invoice item as PurchasedItem

aleksis/apps/tezor/models/invoice.py

@@ -190,7 +190,7 @@ class InvoiceItem(ExtensibleModel):
     )
 
     def as_purchased_item(self):
-        yield PurchasedItem(
+        return PurchasedItem(
             name=self.description,
             quantity=1,
             price=self.price,