CALGARY — The union representing 4,400 flight attendants at WestJet says it has issued 72-hour strike notice, paving the runway for a possible strike during a busy summer travel season. The Canadian Union of Public Employees says the two sides remain far apart on key issues. WestJet, in a statement, said the airline responded to the union’s strike notice by issuing a 72-hour lockout notice. “The decision to issue a lockout notice, in response to the actions taken by the union, was not one that was made lightly. The airline says it operates more than 600 flights per day carrying legions of passengers — more than 70,000 on some days.