By Catarina Demony and Patricia Vicente Rua LISBON (Reuters) – Portugal’s civil aviation workers on Monday threatened to go on a three-day strike in late August, accusing the conglomerate that operates the country’s biggest airports of failing to increase wages and provide better working conditions. The strike threat by Portugal’s Civil Aviation Workers’ Union (SINTAC) and the Commercial Aviation Staff Union (SQAC) is the latest in a series of walkouts at a time Europe’s transport sector continues to struggle handling a return to travel after COVID-19 pandemic lockdowns. The strike is set to t…