Upgrade Python3 to 3.8.5

While #3 (closed) can certainly be done with the current Python 3.7 built by Omnibus, it would still be a step backwards from the current 3.8.5 installed by Heptapod Docker in the final images for Python3.

We are running Python 3.8 for the whole in production on several source installations, including the present one, so the risk of unexpected problems beside packaging issues is quite low.