PROVIDENCE — A Warwick man who pleaded guilty to possessing child pornography was sentenced Wednesday to 42 months in federal prison, according to the office of U.S. Attorney Aaron L. Weisman.Prosecutors say that 34-year-old Stephen P. Langlois Jr. used bitcoin to to subscribe to an overseas-based child pornography website, where he acquired more than 100 videos, some of which included sadistic depictions of sexual activity.