(Reuters) – Ukraine was formally accepted as a candidate for European Union membership on Thursday, a boost for morale in the war-weary country as Russian assaults wear down the defenders of two cities in the eastern Donbas region. FIGHTING * Ukrainian troops may need to retreat from the eastern frontline city of Lysychansk to avoid encirclement, Luhansk regional governor Serhiy Gaidai said. * Ukraine said its troops were still holding out in Sievieorodonetsk, across the river. * In Ukraine’s south, three cruise missiles hit the port city of Mykolaiv, while two more were shot down near Odesa, …