Bugfix setup of virtual environment

The new version of opencv-python (as part of our requirements.txt) proves to be incompatible with old protobuf which is available Stages/2019a. Thus, it has become impossible to create a new virtual environment on Juwels and HDF-ML. The workaround is to tell pip explicitly to install opencv-python version 4.2.0.34 which also has been used in previous days.