Make the payment term fields readonly for non draft amendment lines

Closes #14746
(grafted from 85902c3ab347d29e9b4e957828add75f2cb2beef)

--HG--
branch : 6.0

modules/purchase_amendment/purchase.py

@@ -195,6 +195,7 @@ class AmendmentLine(ModelSQL, ModelView):
     payment_term = fields.Many2One(
         'account.invoice.payment_term', "Payment Term",
         states={
+            'readonly': Eval('state') != 'draft',
             'invisible': Eval('action') != 'payment_term',
             },
         depends=['action'])

modules/sale_amendment/sale.py

@@ -211,6 +211,7 @@ class AmendmentLine(ModelSQL, ModelView):
     payment_term = fields.Many2One(
         'account.invoice.payment_term', "Payment Term",
         states={
+            'readonly': Eval('state') != 'draft',
             'invisible': Eval('action') != 'payment_term',
             },
         depends=['action'])