setup.py: add sdist_gnome command for creating a release tarball for GNOME
setup.py: add sdist_gnome command for creating a release tarball for GNOME

The difference is the lowercase name of the directory and it's a .tar.xz.
xztar requires Python 3 so the command only works with Python 3.

pexip/pygobject,GNOME/pygobject,GNOME/pygobject,GNOME/pygobject,pexip/pygobject,pexip/pygobject