Without these pytest options, the swh namespace is not taken into account in package name when executing tests ouside tox, meaning package variable equals "core.api.tests" in the context of the swh/core/api/tests/test_serializers.py file instead of "swh.core.api.tests", which makes some tests failed.
See https://jenkins.softwareheritage.org/job/DENV/job/run-all-tests/1299/artifact/swh-core.log