Make a copy of the payment iterators

We copy the iterators created by groupby in order to prevent consumption of the
underlying iterator.

issue3660
review3321002
(transplanted from f6a75fb9052b5a3650a4230233d4dd6d478099b5)

dunning.py

@@ -121,7 +121,8 @@ class Letter(CompanyReport):
                         ],
                     order=[('party', 'ASC'), ('id', 'ASC')]))
         payments = list(chain(*payments))
-        return dict(groupby(payments, attrgetter('party')))
+        return dict((party, list(payments))
+            for party, payments in groupby(payments, attrgetter('party')))
 
     @staticmethod
     def get_payment_amount(payment):