title: too many values to unpack, python, findcontours
body: <p>im working on an OCR project and here I'm doing hand written digit recognition the problem i have is with the function <code>findcontours()</code>:</p>
  
  <pre><code># Import the modules
  import cv2
  from sklearn.externals import joblib
  from skimage.feature import hog
  import numpy as np
  
  # Load the classifier
  clf = joblib.load("digits_cls.pkl")
  
  # Read the input image 
  im = cv2.imread("photo_2.jpg")
  
  # Convert to grayscale and apply Gaussian filtering
  im_gray = cv2.cvtColor(im, cv2.COLOR_BGR2GRAY)
  im_gray = cv2.GaussianBlur(im_gray, (5, 5), 0)
  
  # Threshold the image
  ret, im_th = cv2.threshold(im_gray, 90, 255, cv2.THRESH_BINARY_INV)
  
  # Find contours in the image
  ctrs, hier = cv2.findContours(im_th.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
  
  # Get rectangles contains each contour
  rects = [cv2.boundingRect(ctr) for ctr in ctrs]
  </code></pre>