here's what I did:
downloaded the .zip version of the GitHub repo. expanded that, opened terminal and cd'd to that directory.
ran the following:
pip install virtualenv
brew install python (though i have 2.7 for octoprint, you'd said this requires 3)
allowed that time to install/compile
virtualenv -p python3.7 .env
.env/bin/pip install -r requirements.txt
allowed that to process to completion, then from that same directory attempted to run:
I only get the error listed, nothing else.
For the record i am on macOS Yosemite 10.10.5 on an Intel Mac Mini Late 2009. Octoprint runs perfectly and the iSight camera works with OBS and Facetime on the system (its an OG FireWire 400 Camera hooked up with a Firewire 800 to 400 adapter)