I had the same error. Here was my workflow. I first installed PIL (not Pillow) using
pip install --no-index -f https://dist.plone.org/thirdparty/ -U PIL
Then I found Pillow and installed it using
pip install Pillow
What fixed my issues was uninstalling both and reinstalling Pillow
pip uninstall PIL
pip uninstall Pillow
pip install Pillow