update python version in setup venv

Currently, there is the following command:

export PYTHONPATH=${cur}/../venv_hdfml/lib/python3.6/site-packages:${PYTHONPATH}

but we are using python3.8 from the HPC modules.

• update python version in mlair/HPC_setup/setup_venv_hdfml.sh
• update python version in mlair/HPC_setup/setup_venv_juwels.sh
• check all other installation scripts for updates