deposit: Unstuck buster debian build

As usual, tox and pypi build are fine.

This time the unstable build is fine but the buster build [1] (the one that matters for deployment) fails.

Unstuck it.

• [1] https://jenkins.softwareheritage.org/job/debian/job/packages/job/DDEP/job/gbp-buildpackage/94/consoleFull

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Migrated from T3072 (view on Phabricator)