add cssselect dependency to packaging scripts
For Inkscape 1.2
In today's developer meeting, I was asked to create a merge request with this change to the packaging.
I hope this is all that is required to update Snap, Appimage, Windows builds and the developer dependency script.
The package exists for Msys2 (https://packages.msys2.org/base/mingw-w64-python-cssselect).