add cssselect dependency to packaging scripts

For Inkscape 1.2

My extensions feature branch, extensions!294 (merged), added a dependency on the python package cssselect (a css parser, https://github.com/scrapy/cssselect)

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).