We only need to install it on CI because the python on CI doesnt come
from brew. Normal brewfile based installation will install openssl as a
dependency of python
Pillow wheel currently not available for python 3.9. So tries to install
from source and fails on missing zlib. So install zlib in brew and make
it available via PKG_CONFIG_PATH. Apparently, this wont be necessary
after Pillow 8, but who knows.