I had to do the following to get to a working state (mostly due to pip dependency build errors)
sudo apt install libcairo2-dev libjpeg-dev libgif-dev
sudo apt install libgirepository1.0-dev
sudo apt install libcups2-dev
sudo apt install xvfb
pip3 install xvfbwrapper
I also needed to add --xvfb in agent.sh to make sure xprop wasn't getting called, or else I would hit this error: catchpoint/WebPageTest.agent#458