Updates for bob's 8.0

• Opencv package from the defaults conda channel has a dependency on hdf5.1.10.2 which conflicts with bob's (1.10.6). Therefore opencv is build in bob/conda and we use this package. New opencv version is 4.5.0 (previously was 3.4.2)

• Updates in documentation config for new sphinx version.

• clean-up of dependencies in conda/meta.yaml and requirements.txt.

• Due to a faulty packaging in the opencv package, it is not found by pkg_resources and we can't have it in requirements.txt. Therefore bob.ip.stereo is now NOT pip-installable, however conda-based install still work.