pytest: Remove use of --import-mode=importlib option
It makes the tests fail when executed in a local venv outside tox.
See https://jenkins.softwareheritage.org/job/DENV/job/run-all-tests/1612/artifact/swh-web-client.log
Staging instance, all changes can be removed at any time
It makes the tests fail when executed in a local venv outside tox.
See https://jenkins.softwareheritage.org/job/DENV/job/run-all-tests/1612/artifact/swh-web-client.log