By Pavel Polityuk and Natalia Zinets LVIV, Ukraine (Reuters) -Ukrainian civilians and foreign students clutching bags and rucksacks fled the besieged city of Sumy in buses and cars on Tuesday in the first successful “humanitarian corridor” opened since Russia’s invasion. Ukraine accused Russian forces of shelling one evacuation route – from Mariupol in southern Ukraine – but Sumy’s regional governor said a temporary ceasefire had held long enough for at least two convoys of vehicles to leave the northeastern city. The first evacuation convoy met a column of more than 160 Russian military vehic…