try this
pip3 install -U Pillow -t /media/sda/seafile/seafile-server-latest/seahub/thirdpart
changing the path to your /seahub/thirdpart directory
Try to install as well libopenjp2-7 and libopenjp2-7-dev , they are needed for Pillow in buster
with apt install libopenjp2-7 libopenjp2-7-dev