github actions CI updated to python 3.9

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.

.github/workflows/ci.py

@@ -40,6 +40,9 @@ def install_deps():
     if is_bundle:
         install_bundle()
     else:
+        if is_macos:
+            # needed for zlib for pillow, should not be needed after pillow 8.0
+            os.environ['PKG_CONFIG_PATH'] = '/usr/local/opt/zlib/lib/pkgconfig'
         run('pip3 install Pillow pygments')