build: do not require opencv-python
build: do not require opencv-python

While cv2 is required, it may be provided by other means than using
the pypi opencv-python package.

oscarlorentzon/OpenSfM,oscarlorentzon/OpenSfM,mapillary/OpenSfM,oscarlorentzon/OpenSfM,mapillary/OpenSfM,mapillary/OpenSfM,mapillary/OpenSfM,mapillary/OpenSfM,oscarlorentzon/OpenSfM,oscarlorentzon/OpenSfM