“New York City was scaling back its stop-and-frisk program even before a federal judge ruled in 2013 that the tactics underlying it violated the constitutional rights of minority citizens. It’s hard not to look at marijuana arrests today without thinking of that saga. Although the city has reduced the number of arrests for low-level marijuana possession, black and Latino New Yorkers are far more likely to be arrested for smoking in public than whites, who are just as likely to use marijuana.”