Unstuck swh.loader.mercurial debian build

Build is stuck in buster-swh failing...

(unstable-swh is fine)

most likely hitting [2]

• [1] https://jenkins.softwareheritage.org/job/debian/job/packages/job/DLDHG/job/gbp-buildpackage/52/console

• [2] https://forge.softwareheritage.org/source/swh-loader-mercurial/browse/master/swh/loader/mercurial/hgutil.py$6

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Migrated from T3026 (view on Phabricator)