Michael Buckner/Variety/Getty ImagesUnder Demna’s tenure, Gucci has gone back to its unabashedly sexy roots—and it appears Kim Petras has done the same. Last night, the Problématique singer stepped out to the Los Angeles County Museum of Art, in one of the definitive designs from Demna’s runway debut collection: an otherwise pared-back black dress that put all its emphasis on a leg slit so high it would make Angelina Jolie blush. During a gala c…