conftest: Declare swh.core pytest_plugin

As the other pytest plugin are declared.

The pytest.ini modification is required because it autoloads (as in swh-model!137 (closed) for example).

I tried to stop swh.core from autoloading its fixture but it's apparently more complex than anticipated [1]. I don't want to plunge in there just yet so here goes.

• [1] swh-core!172 (closed)

Test Plan

tox

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Migrated from D4182 (view on Phabricator)