Pin Pillow version < 9.0

Installing a fresh env with Pillow ~v0.9 (latest version) leads to:

from PIL import Image, ImageOps, ImageEnhance, PILLOW_VERSION
ImportError: cannot import name 'PILLOW_VERSION' from 'PIL' 

In the CI, the installed version is still v0.6. Pinning to version < v9.0 solves the issue.