Rental costs increased at the slowest pace in three and a half years in December, a small bit of happy news for consumers in a housing market that’s seemed relentlessly strained for some time. Nationwide annual rent was up 3.5% compared to a year ago, the Labor Department said Friday. While that’s down only one tick from November, it still counts as the slowest yearly increase since June 2015, and continues a two-year trend of slowing rental price growth.