Triggering the flow in such a way is not possible (IMHO).
However, you can set the trigger's recurrence to one hour and check with the Get item's Filter Query if the ExpiryDate is within the hour.
Another way to archive your goal would be to chose a trigger "When an item is created or modified" and use the Delay until action.
After the delay, you could check, if the ExpiryDate has changed and send the mail, if not.
Keep in mind that this has some limitations (e.g. flow duration limit)