You almost did it. You were tricked by the fact that abs(imagem-255) will give a wrong result since your dtype is an unsigned integer. You have to do (255-imagem) in order to keep the integers unsigned:
def inverte(imagem, name):
imagem = (255-imagem)
cv2.imwrite(name, imagem)
You can also invert the image using the bitwise_not function of OpenCV:
imagem = cv2.bitwise_not(imagem)