CHICAGO — A trip to the supermarket in Dolton turned fatal this weekend when a 3-year-old boy found a gun in the back seat of a vehicle, picked it up and accidentally shot his mother in the neck, killing her. Dejah Bennet, 22, was pronounced dead at University of Chicago Medical Center at 9:34 p.m. Saturday, according to information from the Cook County medical examiner’s office. She was rushed to the hospital after emergency responders located her suffering from a gunshot wound at the Food 4 Less Fuel Center, 1000 E. Sibley Blvd. in Dolton, officials said. Andrew Holmes, a trustee for the cit…