title: Pillow: conversion from RGB to LAB not supported
body: <p>I am converting a image to Lab color space from RGB color space using </p>
  
  <p><code>image.convert('LAB')</code></p>
  
  <p>But raises the error</p>
  
  <p><code>ValueError: conversion from RGB to LAB not supported</code></p>
  
  <p>Converting it to other color spaces (for example HSV, YCbCr, LA) works fine but raises the error for the case of LAB. I know I can achieve this using skimage or by using ImageCms, but I am wondering why this method is not working?</p>