This function can be used if no scipy is availabe.
        It's 7 times slower though.

def quantize_without_scipy(self, image):
        """" This function can be used if no scipy is availabe.
        It's 7 times slower though.
        """
        w, h = image.size
        px = np.asarray(image).copy()
        memo = {}
        for j in range(w):
            for i in range(h):
                key = (px[i, j, 0], px[i, j, 1], px[i, j, 2])
                try:
                    val = memo[key]
                except KeyError:
                    val = self.convert(*key)
                    memo[key] = val
                px[i, j, 0], px[i, j, 1], px[i, j, 2] = val
        return Image.fromarray(px).convert("RGB").quantize(palette=self.palette_image())