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

 [pytest]
 norecursedirs = build docs .*
 asyncio_mode = strict
-addopts = "--import-mode=importlib"
+consider_namespace_packages = true

requirements-test.txt

 msgpack
-pytest
+pytest >= 8.1
 pytest-mock
 pyyaml
 swh.core[testing] >= 3.0.0