By Tom Balmforth and Pavel Polityuk KYIV (Reuters) -Ukrainian troops said on Saturday they had taken 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 capture came just a day after Putin proclaimed the annexation of four Ukrainian regions – including Donetsk, where 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. The Ukrainian soldiers made the claim in a vid…