Using with OpenCV v4.2

[tomoyk]

On OpenCV 4.0 or later version, findContours() return 2 values.

https://docs.opencv.org/4.3.0/d3/dc0/group__imgproc__shape.html#gadf1ad6a0b82947fa1fe3c3d497f260e0

I fixed codes below.

# BEFORE
image, contours, hierarchy = cv2.findContours(mask, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

# AFTER
contours, hierarchy = cv2.findContours(mask, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)