pytest: Fix tests execution with pytest 8.1

Remove use of --import-mode=importlib pytest option and use
new option consider_namespace_packages to fix tests execution
with latest pytest release.

pytest.ini

@@ -3,6 +3,6 @@ addopts =
   # we need this to avoid conflicts between the autoloaded fixture,
   # and the manually loaded one.
   -p no:pytest_swh_scheduler
-  --import-mode=importlib
 norecursedirs = build docs .*
 asyncio_mode = strict
+consider_namespace_packages = true

requirements-test.txt

 pandas
-pytest
+pytest >= 8.1
 pytest-mock
 requests_mock
 swh-scheduler[testing]