SEASON PREMIERE: Eight new episodes of this history-on-the-fly political nonfiction series appear on Showtime, beginning with tonight’s season premiere. Mark McKinnon, John Heilemann, and Alex Wagner return. And their attention, this week, is partly on the Democratic hopefuls for the 2020 presidential race, and partly on the latest scandal embroiling our current President. Just in time for the Circus premiere, this week it’s a big one, involving Ukraine, the U.S. intelligen