Black Widow gets a lot better after the first act. Mostly because the prologue—set in 1995 Ohio where tween-who-will-be-Scarlet-Johansson Ever Anderson lives with her All-American family (little sister Violet McGraw, mom Rachel Weisz, dad David Harbour)—is almost classy enough. With better music and a more patient, less blandly jingoistic look at Americana, it’d be potentially…