By Ann Hornday, The Washington Post Three stars. Rated PG-13. 84 minutes. Like the warm summer day it chronicles, “Southside With You” possesses a mellow, languorous vibe, an infectious easygoing charm that insinuates itself gently, then seductively, as the couple at its center experiences the stirrings of what might be true love. That the two young people in question are named Barack Obama and Michelle Robinson almost feels like an afterthought in Richard Tanne’s modest but enormously enjoyable throwback of a movie.