title: Warning: QT_DEVICE_PIXEL_RATIO is deprecated
body: <p>I am using matplotlib alongside openCV to plot some thresholding methods and the following warning with no output is thrown:</p>
  
  <blockquote>
    <p>Warning: QT_DEVICE_PIXEL_RATIO is deprecated. Instead use:<br>
    QT_AUTO_SCREEN_SCALE_FACTOR to enable platform plugin controlled
    per-screen factors.    QT_SCREEN_SCALE_FACTORS to set per-screen
    factors.    QT_SCALE_FACTOR to set the application global scale
    factor.</p>
  </blockquote>
  
  <p>I am using Ubuntu 19.04</p>
  
  <pre><code>import cv2 as cv
  from matplotlib import pyplot as plt
  
  img = cv.imread('gradient.jpg', 0)
  
  _, th1 = cv.threshold(img, 50, 255, cv.THRESH_BINARY)
  _, th2 = cv.threshold(img, 200, 255, cv.THRESH_BINARY_INV)
  _, th3 = cv.threshold(img, 200, 255, cv.THRESH_TRUNC)
  _, th4 = cv.threshold(img, 127, 255, cv.THRESH_TOZERO)
  _, th5 = cv.threshold(img, 127, 255, cv.THRESH_TOZERO_INV)
  
  titles = ['Original Image', 'BINARY', 'BINARY_INV', 'TRUNC', 'TOZERO', 'TOZERO_INV']
  images = [img, th1, th2, th3, th4, th5]
  
  for i in range(6):
      plt.subplot(2, 3, i + 1), plt.imshow(images[i], 'gray')
      plt.title(titles[i])
      plt.xticks([]), plt.yticks([])
  </code></pre>