By Pavel Polityuk and Natalia Zinets KYIV (Reuters) -Russia resumed its assault on the last Ukrainian defenders holed up in a giant steel works in Mariupol on Saturday, days after Moscow declared victory in the southern city and said its forces did not need to take the plant. Ukrainian President Volodymyr Zelenskiy said the country’s army was not ready to try to break through the siege of the port city. But he said America’s top diplomat, Secretary of State Antony Blinken, and U.S. Defense Secretary Lloyd Austin would visit Kyiv on Sunday and discuss the types of weapons Ukraine needs to battl…