[FIX] payment: Keep session serializable

Fix error like "TypeError: {4} is not JSON serializable" when
serializing the session object (e.g. for external storage)

closes odoo/odoo#41341

X-original-commit: e91e38fa
Signed-off-by: Damien Bouvy (dbo) <dbo@odoo.com>

addons/payment/controllers/portal.py

@@ -35,7 +35,7 @@ class PaymentProcessing(http.Controller):
         if not transactions:
             return False
         tx_ids_list = set(request.session.get("__payment_tx_ids__", [])) | set(transactions.ids)
-        request.session["__payment_tx_ids__"] = tx_ids_list
+        request.session["__payment_tx_ids__"] = list(tx_ids_list)
         return True
 
     @staticmethod