Python 3.6 installation stopped working

[bertsky]

There seem to have been upstream changes recently that prevent our usual deployments for Python version 3.6, with libgeos (which we need for shapely in ocrd_validators) being the culprit. See here for a CI failure example.

I'm not sure why our CircleCI does not even bother to use the make deps-ubuntu rule. But since it did work in the past, and still does for Python>=3.7, I wonder if we should just modify the test-python36 config to include an apt-get update && apt-get install -y libgeos-dev.