swh-graph: fix CI

swh-graph CI went red a couple of days ago, probably due to some unrelated changes in other packages, but need fixing anyway

https://forge.softwareheritage.org/source/swh-graph/history/master/

a recent failing run is at: https://jenkins.softwareheritage.org/job/DGRPH/job/tests/638/console

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Migrated from T2642 (view on Phabricator)