Define a function named posterize. This function expects an image and a tuple of RGB values as arguments.

def posterize(img, rgb_tuple):
    """Given a tuple of RGB values and an image, returns a copy of the image
    with each color replaced by the corresponding values from the tuple."""
    new_img = img.copy()
    for pixel in new_img:
        for i in range(3):
            pixel[i] = rgb_tuple[i]
    return new_img