Score one more for the heatwave. Metro trains running less frequently, air conditioners failing under the pressure, and now, the cruelest cut of all: Sleater-Kinney fans denied on the D.C. stop of their (Please Don’t Call It A) Farewell Tour. Arriving at a show to find a fire truck parked by the front door and firemen looking sternly into an open manhole can never be a good sign. (A manhole cover that, we’ve heard, housed…