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)