Since version 5.6, `hypothesis` warns on `@given` + function-scoped fixtures.
As a consequence a LOT of warnings are now issued when running `swh-web` tests,
see https://jenkins.softwareheritage.org/job/DWAPPS/job/tests-on-diff/29/console
for instance.
So turns concerned `swh-web` fixtures from function-scoped to module-scoped
(there is strictly no difference in tests execution result after that change)
and ignore some warnings related to external function-scoped fixtures that
are safe to use with `@given` (`client` from `pytest-django` and `mocker` from
`pytest-mock`).