Django extensions graph_models command fails not finding pygraphviz when it's already installed

I have a django project on github that I’m building with circleci. One of the circle.yml lines looks like this:

- (cd fish && ./ graph_models --pygraphviz --all -o $CIRCLE_ARTIFACTS/erd.pdf)

I’ve also tried it this way:

- (cd fish && ./ graph_models --all -o $CIRCLE_ARTIFACTS/erd.pdf)

No matter how I try it it fails with:

CommandError: You need to install pygraphviz python module

I’ve used this exact stack on other circleci projects without issue, so I know it works. The dependency referenced in the failure is clearly installed by pip in the dependencies:

Requirement already satisfied (use --upgrade to upgrade): pygraphviz==1.3.1 in /home/ubuntu/virtualenvs/venv-3.5.2/lib/python3.5/site-packages (from -r requirements.txt (line 65))

How can I get my graph_models to build?