When Barack Obama took a summer job at a Chicago law firm after his first year of law school, a recent Harvard Law School graduate named Michelle Robinson was assigned to be his supervisor. He asked her to come with him to a community meeting and she agreed, insisting that it was not a date. But by the end of the day, they saw Spike Lee’s “Do the Right Thing” and shared ice cream — and a kiss. “Southside With You” is the story of that date, opening in August.