In the devilfish-flotilla directory run
virtualenv venv
This creates a directory venv that holds the virtual environment's data. To enter the virtual environment run
source venv/bin/activate
We can see that we are inside the virtual environment because the shell prompt has a (venv) prefix. Now, install the necessary software.
pip3 install sphinx
To leave the virtual environment run
deactivate
In the virtual environment run:
make html