test_serializers: Move to pytest for that specific erratic assertion

Ideally we should refactor those entirely.

Should definitely fix the debian build [1]

• [1] https://jenkins.softwareheritage.org/job/debian/job/packages/job/DCORE/job/gbp-buildpackage/118/console

rDCORE84c7319bc37abb5fd97af994a9527ae01218ee44 was not enough.

Got mislead by pytest's better raises function apparently. Although neither tox nor pytest pick it up... i don't get why but meh... Let's unstuck the build.

Test Plan

tox

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Migrated from D3460 (view on Phabricator)