Using opencv-python-headless 4.6.x we get the following error when
detecting the contours of the detected text regions:
module 'cv2' has no attribute 'cv2'
Work around this by requiring OpenCV < 4.6 for now. See also the
discussion at
https://github.com/qurator-spk/sbb_textline_detection/issues/60.
Fixes gh-60.
- Border: intersect with page frame
- text regions: intersect with (new) Border
- text lines: intersect with (new) text region
(and back-transform at all)