Candice Bergen hatched the idea for a sequel to “Book Club” before the first had even come out. She decided that she, Jane Fonda, Mary Steenburgen and Diane Keaton would have to go to Italy. The dream came true. “Book Club: The Next Chapter,” out Friday, finds the women traveling to Rome, Venice and Tuscany. It’s part bachelorette party for Fonda’s character, who is getting married for the first time (to Don Johnson) and part bucket list fulfillment. Years ago, life had gotten in the way of a trip, and post-pandemic they’re all feeling a little more adventurous.