Get rid of the arrow library

The arrow date and time management library is still used in the swh.scheduler package. For the sake of consistency and simplicity, we must get rid of it in favor of pure datetime objects.

Once done, the swh.core package can be updated to drop support for arrow datetime objects in the serializers module.

This should help reducing the number of custom converters.

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Migrated from T2835 (view on Phabricator)