Former “Sex and the City” star Cynthia Nixon (Miranda) said on Twitter Monday that she’ll challenge Gov. Andrew Cuomo in New York’s Democratic primary in September. Her announcement sets up a race pitting an openly gay liberal activist against a two-term incumbent with a $30 million war chest and possible presidential ambitions. “Nixon has her work cut out for her. A Siena College poll released Monday showed Cuomo leading her by 66% to 19% among registered Democrats, and by a similar margin among self-identified liberals. Nixon did a little better among younger and upstate Democrats, but didn’t have more than a quarter of either group.