Failure with OpenCV 4

Using opencv-python-4.1.0.25, the following error is generated:

```
$ python pyANPD.py image.jpg
Traceback (most recent call last):
File "pyANPD.py", line 152, in
o1 = process_image(path, 0, type='rect')
File "pyANPD.py", line 79, in process_image
_,contours,_=cv2.findContours(gray, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_NONE)
ValueError: need more than 2 values to unpack
```

This does not happen with OpenCV 3.4.5.20, i.e. use
`pip install opencv-python==3.4.5.20`