A very routine twelve rounds of tragedy, resilience and redemption, the boxing film “Southpaw” is a conventionally told dramaturgy high on intensity, but low on human insight or novel ways to tell a familiar story. While the word southpaw has become synonymous with being a lefty, in loose terms, southpaw boxing is slightly more complex: it means a pugilist who does not fight in the “orthodox stance.” Which is ultimately ironic since there is almost absolutely nothing unconventional (or complex, really) about, “Southpaw,” a heavy-handed comeback story limited by its reliance on over-the-top melodrama. Originally written as an acting vehicle for rapper Eminem by Kurt Sutter (the creator of “Sons of Anarchy”), while the story evolved over the years, what remains is trailer-trash sensibilities and hip-hop swagger characteristic of Marshall Mathers' hot-tempered proclivities, a thuggish mien of volatility and stubborn, poor choices that frustratingly mar the movie’s well-meaning but...