Stovetop Spinach Basil Pesto Mac and Cheese

dry elbow pasta 1 stick (1/2 cup) unsalted butter 1/2 cup all-purpose flour 3 cups milk 2 cups shredded white cheddar cheese 1/2 teaspoon kosher salt 1/4 teaspoon black pepper 2-3 Tablespoons Spinach Basil Pesto, or other pesto