title: Color space conversion with cv2
body: <p>I'm migrating from <code>cv</code> to <code>cv2</code> and I'm having difficulties finding an equivalent of the <code>cv.CvtColor</code> function.</p>
  
  <p>I read in the documentation that <code>cv2.cvtColor</code> existed but I don't know how to use the third parameter.</p>
  
  <p><code>cv2.CV_BGR2Lab</code> doesn't exist. Neither does <code>cv2.CV2_BGR2Lab</code> and when I use <code>cv.CV_BGR2Lab</code> all I get is an error: </p>
  
  <p><code>cv2.cvtColor(img, img, cv.CV_BGR2Lab)</code></p>
  
  <p><code>TypeError: only length-1 arrays can be converted to Python scalars</code></p>
  
  <p>A simple piece of code that produces the problem:</p>
  
  <pre><code>img = cv2.imread(path)
  cv2.cvtColor(img, img, cv.CV_BGR2Lab)
  </code></pre>