pytest.ini: Avoid loading flask plugin to prevent fixture name clash

pytest-django and pytest-flask both declare a fixture named client and the wrong one is picked when running swh-deposit tests.

So avoid the loading of pytest flask plugin to prevent errors when running tests.

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Migrated from D3051 (view on Phabricator)

pytest.ini

 [pytest]
+addopts = -p no:flask
 norecursedirs = docs
 DJANGO_SETTINGS_MODULE = swh.deposit.settings.testing