title: OpenCV Python: Convert RGB to YCrCb
body: <p>I can't seem to convert RGB to YCrCb in the new OpenCV python API (cv2). When I run this code:</p>
  
  <pre><code>img = cv2.imread('img1.jpg')
  imgYCC = cv2.cvtColor(img, cv2.COLOR_RGB2YCrCb)
  </code></pre>
  
  <p>, I get this error:</p>
  
  <pre><code>AttributeError: 'module' object has no attribute 'COLOR_RGB2YCrCb'
  </code></pre>
  
  <p>What am I doing wrong? </p>