Run "./pre-inst-env guix lint hydrus-network", it will have a remark
about this.  Also, technically this is racy -- it's possible for
python to start before Xvfb is ready though so far this doesn't seem
to have caused trouble for other packages yet AFAIK -- I recommend
"xvfb-run" "--" "python" "test.py" instead.

I though I missed something so I ran lint again, but it only said there's a new version of hydrus available.
Nevertheless I changed the invocation to what you recommended. I also updated hydrus to version 493.

Looks like the linter ignored hydrus-network because it doesn't understand the 'let' construct ... work for later (not this patch series) ...

Greetings,
Maxime.