swh: Restore __init__.py file
Tests are failing otherwise when pytest is not invoked through tox as swh is no longer included in module paths.
See https://jenkins.softwareheritage.org/job/DENV/job/run-all-tests/1299/artifact/swh-core.log
Tests are failing otherwise when pytest is not invoked through tox as swh is no longer included in module paths.
See https://jenkins.softwareheritage.org/job/DENV/job/run-all-tests/1299/artifact/swh-core.log