title: No module called cv2
body: <p>I am using Ubuntu 18.04 and I follow the official install instruction of OpenCV.
  When I tried to import <code>cv2</code> on python3.8, it shows</p>
  <pre><code>No module named cv2 
  </code></pre>
  <p>but for python2.7, it works for me and I use the command <code>cmake ../</code> to access and it only shows python2.7 has built into it. Does anyone meet the same problem? Please see the screenshot for more details.</p>