Upgrade to supported base image and Python version

[ianhinder]

Dependabot doesn't support our old Python version anymore, so we need to upgrade.

[ianhinder]

In the process of upgrading everything, I ran into an incompatibility with pygraphvis. The GV_FILE_PATH variable has been removed. This was some sort of sandboxing mechanism. Some comments on it here. I think it should be possible to pass the image_dir for graphviz directly to the graphvis method, but my initial attempt didn't work.