There was several issues with this method:
- the workflow is incorrectly called (`purchase.signal_workflow(...)` instead)
- the workflow no longer exists (`purchase.state = 'sent'` instead)
- the code is only triggered when sending an actual mail and not a notification
- the code is never executed as the change is already changed
The last point is due to the override of `send_mail` in mail.compose.message.
The state is changed before `_postprocess_sent_message` is called (if called at
all).
Similar change of state is already done for sale.order.
This commit replaces and closes#14760
Martin Trigaux