title: " No module named 'cv2' " but it is installed
body: <p>I have installed the whl file with include opencv + contribution because i want to use the SIFT-algorithm. I installed it with pip in my conda environment, so when i tipp in "conda list" it shows me 
  "opencv-python             3.4.5+contrib             "
  But when I try to run "ModuleNotFoundError: No module named 'cv2'". My Project is also using the correct environment </p>
  
  <pre><code>import cv2
  sift = cv2.xfeatures2d.SIFT_create()    
  ModuleNotFoundError: No module named 'cv2'
  </code></pre>