I was able to fix this by upgrading pip.
For anyone interested, pip issue: TypeError: expected string or bytes-like object - When doing pip install, or upgrading pip. · Issue #11343 · pypa/pip · GitHub
And the fix:
curl https://bootstrap.pypa.io/get-pip.py
sudo python3 get-pip.py
sudo because the sixfab script uses sudo when installing atcom
If you want to use the same version of pip without sudo then use the revised command:
curl https://bootstrap.pypa.io/get-pip.py
sudo python3 get-pip.py
python3 get-pip.py
But you’d be better off using a virtualenv for libs that don’t require sudo