Similarly to Chris’ comment on the bug, I’m able to follow the steps that you outlined successfully:
python3 -m venv test-libjuju
source ./test-libjuju/bin/activate
pip install juju
python3
>>> from juju.model import Model
I’m running Python 3.8.2, and I did a which pip before running pip install to verify that I was getting the pip from my venv, and not a system wide pip.
You might take a look at the site-packages within your venv to verify that juju successfully installed.