The Steppenwolf Theatre Company’s current revival of David Mamet’s 1975 play American Buffalo, in residence at Princeton’s McCarter Theater until March 28, is, at once, claustrophobic and explosive, both testosterone-soaked and, surprisingly, deeply sentimental. Mamet, whose plays often pick at scabs on the American psyche, explores the shifting and unsteady sand that lies beneath the relationships between three low-life grifters as they try to plan a robbery.