By Kyle Wood The Celtics are the NBA championship favorites at SI Sportsbook, slightly ahead of last year’s champion Warriors. NBA training camps open across the league Tuesday and betting lines are already available for the Oct. 18 opening night doubleheader. Basketball is oh-so close to being back. It’s been a rather eventful offseason, even by the NBA’s theatrical standards, with plenty of trades and transactions and, in some cases, lack thereof, that had outsized impacts on the 2023 championship race. Perhaps most importantly, Kevin Durant is still in Brooklyn, buoying the Nets’ title hope…