To find missing dependencies after sudo pip3 install opencv-contrib-python, use this link https://blog.piwheels.org/how-to-work-out-the-missing-dependencies-for-a-python-package/
With a fresh install of Raspbian Buster, you need (with apt install):
- libhdf5-103
- libatlas3-base
- libjasper1
- libqtgui4
- libqt4-test