pytest.ini: Ensure '--import-mode importlib' option is always used

Fix hanging test when executed outside tox.

pytest.ini

 [pytest]
-# We need this to avoid conflicts between the autoloaded fixture, and the
-# manually loaded one.
-addopts = -p no:pytest_swh_scheduler
+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

tox.ini

@@ -13,7 +13,6 @@ deps =
   pytest-cov
 commands =
   pytest --doctest-modules \
-         --import-mode importlib \
          --rootdir {envsitepackagesdir} \
          --cov={envsitepackagesdir}/swh/lister \
          --cov-branch \