title: python OpenCv IMREAD_UNCHANGED only return three channels
body: <p>Hi guys I'm trying to figure out what's wrong with my code </p>
  
  <p>I want to load my image contains alpha channel </p>
  
  <p>the description from official website says that: </p>
  
  <blockquote>
    <p>cv.IMREAD_UNCHANGED: If set, return the loaded image as is (with alpha channel, otherwise it gets cropped).</p>
  </blockquote>
  
  <p>Here's my try:</p>
  
  <pre><code>import cv2 as cv 
  
  img2 = cv.imread( 'lbj.jpg' , cv.IMREAD_UNCHANGED)
  
  img2.shape
  </code></pre>
  
  <p>And the result shows : <code>(350, 590, 3)</code></p>
  
  <p>Isn't is supposed to be <code>(350,590,4)</code>?</p>
  
  <p>Thanks!</p>