test_storage: Force LazyContent __type__ attribute to be serializable

This actually fixes the debian build package failure [1]

I do not like it much as i'm fooling the extra serializer into my bidding. I do not see a better way though, ideas welcome.

Also, i do not get why the tox build do not need it...

• [1] https://jenkins.softwareheritage.org/view/Debian%20packages/job/debian/job/packages/job/DSTO/job/gbp-buildpackage/137/console

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Migrated from D2737 (view on Phabricator)