Advance python version to 3.9 to build docker in Ubuntu Lunar Lobster

[vaclavhanzl]

Advance python version from 3.7 to 3.9 at multiple places in the code. This fixes the docker build with Ubuntu Lunar Lobster which became "Latest" a few days ago.

(I noticed a likely broken docker build from main in my other PR #308. See it for more detailed comments.)

[vaclavhanzl]

Looking at our Dockerfile, I am no longer sure how exactly (and whether at all) was the problem caused by the new Ubuntu release. I am nearly sure I've seen dependency on the "latest" Ubuntu somewhere (in log files?) but our Dockerfile says "FROM nvidia/cuda:11.3.1-cudnn8-runtime-ubuntu18.04" which pinpoints things very exactly.
Anyway, this PR fixes the problem but we may want to understand the exact chain of events breaking our build to prevent future problems.

[christinaflo]

Okay merged this in, thank you! I am also not sure what caused this, although I do see in the image history that it's been updated within the last 5 months.

[vaclavhanzl]

It's worth some ghost hunting. Unlike the Colab where we sort of change the airplane in flight with our tricky conda install and some fragility can be expected, the Docker build is expected to have dependencies well frozen - but somehow changes leaked in. I might have a detailed look.

[vaclavhanzl]

Oh, one rather urgent thing @christinaflo - if the Docker image is pushed somewhere and NOT just made for the sake of the PR test and discarded, we should increase the image version?
I thought the Docker image is made and discarded. (There is an issue #73 which proposed to push the resulting image to Dockerhub but this issue was never closed.)