Classic Coleslaw

finely chopped cabbage (about 1 head)1/4 cup shredded carrot (1 medium carrot)2 tablespoons minced onion1/3 cup granulated sugar1/2 teaspoon salt1/8 teaspoon pepper1/2 cup mayonnaise1/4 cup buttermilk1/4 cup cider vinegar1 teaspoon celery seed