Adapt visit_date type from string to datetime
This follows the latest loader changes. Now we deal only with visit_date as datetime within the loader.
This should fix the master build which should fail at some point when triggered [1]
Related to T2310
- [1] (manually triggered) https://jenkins.softwareheritage.org/job/DLDG/job/tests/687/console
Test Plan
tox
Migrated from D3272 (view on Phabricator)