Comment 2 for bug 1871530

Found in ./DistUpgradeViewGtk3.py:

        # if no libgtk2-perl is installed show the terminal
        frontend= os.environ.get("DEBIAN_FRONTEND") or "gnome"
        if frontend == "gnome" and self._cache:
            if (not "libgtk2-perl" in self._cache or
                not self._cache["libgtk2-perl"].is_installed):
                frontend = "dialog"
                self.expander.set_expanded(True)
        self.env = ["VTE_PTY_KEEP_FD=%s"% self.writefd,
                    "APT_LISTCHANGES_FRONTEND=none"]
        if "DEBIAN_FRONTEND" not in os.environ:
            self.env.append("DEBIAN_FRONTEND=%s" % frontend)

libgtk2-perl is obsolete and no longer installed by default on the desktop. This needs updated for libgtk3-perl.