So meanwhile I have again tried to get 7.1.3 to work by installing pyenv and deploying 3.6.10.
python -V
Python 3.6.10
afterwards I have installed python libraries for that version again according to install doc.
Problem I have now is that seahub fails to start without any error message in the log.
Looks like I have totally screwed up. I wish I could fix this easily.
thanks for your attempt to help!