tests: Simplify setup after pytest 8.1 release

Since the release of pytest 8.1, some pytest options are no longer
needed and editable install can be used when running tests using tox.

pytest.ini

 [pytest]
-addopts = -p no:flask --import-mode=importlib
+addopts = -p no:flask
 norecursedirs = build docs .*
 asyncio_mode = strict
 DJANGO_SETTINGS_MODULE = swh.auth.tests.django.app.apptest.settings
+consider_namespace_packages = true

requirements-test.txt

 django-stubs
 djangorestframework-stubs
-pytest
+pytest >= 8.1
 pytest-django
 pytest-mock
 requests_mock

tox.ini

@@ -7,6 +7,7 @@ envlist =
   py3
 
 [testenv]
+use_develop = true
 extras =
   testing
   django
@@ -15,14 +16,10 @@ deps =
   pytest-cov
 commands =
   pytest --doctest-modules \
-         --rootdir {envsitepackagesdir} \
-         --cov={envsitepackagesdir}/swh/auth \
+         --cov=swh/auth \
          --cov-branch \
-         {envsitepackagesdir}/swh/auth \
+         swh/auth \
          {posargs}
-# --rootdir (with --import-mode from pytest.ini) are required to make tests
-# that depends on the test file to be a proper submodule of the swh namespace
-# after migration to PEP420 (implicit namespace).
 
 [testenv:black]
 skip_install = true