setup.py: Declare storage's pytest-plugin module
This should avoid having to adapt debian builds all around.
Somehow, the /conftest.py which declares it on other modules depending on it is either no longer part of the build and/or even ignored by new pytest...
So this looks like the right way to stay DRY. Also that unifies with what the swh.core is declaring regarding such pytest-plugin. Without any needs for other modules to declare it (in their respective build).
Otherwise, i need to adapt all packages' debian/rules to add
export PYTEST_PLUGINS=swh.storage.pytest_plugin[,...whatever else...](which i did on storage and on scrubber too so far to unstuck builds but that's not DRY to me...)
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