By Jonathan Saul LONDON (Reuters) – An attack by assailants on a container ship in the Suez canal has heightened risks for merchant shipping using the vital waterway, with further threats considered likely as political turmoil continues in Egypt. On Sunday an army source said three people had been arrested after opening fire with machine guns on a Chinese-owned container ship passing through the canal, a big source of revenue for Egypt, playing down what the Suez Canal Authority’s chief had described earlier as a terrorist attack. …