WASHINGTON—Despite calls from some liberal activists for Justice Sonia Sotomayor to step down while Democrats can fill her seat before Inauguration Day, she has no plans to retire from the Supreme Court, people close to the justice said. “This is no time to lose her important voice on the court. She just turned 70 and takes better care of herself than anyone I know,” said one person close to the justice, suggesting that progressives turn their attention to other ways of safeguarding the Constitution after President-elect Donald Trump takes office.Sotomayor, appointed in 2009 by then President Barack Obama, is the...