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...)

[1] 2c311af8

[2] swh-scrubber@8f3dad71