Next to requirements.txt one also needs to install tesseract

[stefan52a]

For Ubuntu that would mean:

sudo apt install libtesseract-dev libleptonica-dev liblept5
sudo apt-get install tesseract-ocr-all
pip install tesseract

[stefan52a]

I think
https://pillow.readthedocs.io/en/latest/installation.html
is also needed