By Tom Balmforth and Pavel Polityuk KYIV (Reuters) -Russia said on Saturday its troops had abandoned the key bastion of Lyman in occupied eastern Ukraine, a stinging defeat that prompted a close ally of President Vladimir Putin to call for the possible use of low-grade nuclear weapons. The announcement came just a day after Putin proclaimed the annexation of four Ukrainian regions – including Donetsk, where the city of Lyman is located – and placed them under Russia’s nuclear umbrella, at a ceremony that was condemned by Kyiv and the West as an illegitimate farce. “In connection with the creat…