The linkcheck started to fail because of a new check on gitlab.gnome.org
that now forbids Sphinx User-Agent, returnin 406 HTTP status.
( chapter10: line 115) broken https://gitlab.gnome.org/GNOME/libxml2/-/wikis/home - 406 Client Error: Not Acceptable for url: https://gitlab.gnome.org/GNOME/libxml2/-/wikis/home
Backport of MR !11747
Merge branch 'backport-mnowak/linkcheck-set-user-agent-9.20' into 'bind-9.20'
See merge request isc-projects/bind9!11748