model: Fix "unused 'type: ignore' comment" error with mypy 0.782
Jenkins job for swh-model is currently failing because of that (see https://jenkins.softwareheritage.org/job/DMOD/job/tests/).
Changes in that diff also guarantee that no typing errors will be raised when using mypy<0.782
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Migrated from D3840 (view on Phabricator)