CALGARY — A union representing 4,400 flight attendants at WestJet has issued 72-hour strike notice. The Canadian Union of Public Employees says the airline then issued a 72-hour lockout notice. While giving notice of a strike does not mean flight attendants will hit the picket lines, a strike could begin as soon as Sunday if the two sides don’t reach a deal. WestJet has been allowing passengers with flights scheduled any time between July 30 and Aug. 4 to make a one-time change or cancel it with no fee. The airline says it operates more than 600 flights per day, carrying more than 70,000 passengers on some days.