Not compatible with open-cv < 4.0.0

Hello there!

Great tool. Thank you for making it public :) . I noticed that the tool won't work with open-cv < 4.0.0. In particular, this line: https://github.com/qurit/rt-utils/blob/main/rt_utils/image_helper.py#L90 will return an error because cv.findContours returns a tuple of three elements in open-cv < 4.0.0 (e.g., see -> https://docs.opencv.org/3.4.17/d3/dc0/group__imgproc__shape.html#ga95f5b48d01abc7c2e0732db24689837b for 3.4.17). That behaviour changed in 4.0.0.

I am not sure if there are other backward compatibility issues.

I would suggests updating the requirements file to enforce a version of open-cv >= 4.0.0 (or whichever version the tests were run on) so that if the existing python environment already contains an old version of open-cv, this gets also updated.

My best regards,

P