Sort SEPA payments by date

In order to compute the proper sequence type, the payments must be ordered by
ascending date in case there are many payments for a first used mandate.

Closes #12614
(grafted from 2c9566afb532ea96c728c16b9c3fe965fffa93e7)

--HG--
branch : 6.0

modules/account_payment_sepa/payment.py

@@ -5,6 +5,7 @@ import os
 import unicodedata
 from itertools import groupby
 from io import BytesIO
+from operator import attrgetter
 
 import genshi
 import genshi.template
@@ -208,8 +209,9 @@ class Group(metaclass=PoolMeta):
         pool = Pool()
         Payment = pool.get('account.payment')
         if self.kind == 'receivable':
+            payments = sorted(self.payments, key=attrgetter('date', 'id'))
             mandates = Payment.get_sepa_mandates(self.payments)
-            for payment, mandate in zip(self.payments, mandates):
+            for payment, mandate in zip(payments, mandates):
                 if not mandate:
                     raise ProcessError(
                         gettext('account_payment_sepa'