Make pytest plugin optional and provide it in a separate extra requires

This allows tests that require the pytest plugin to depend on
`swh.journal[pytest]` rather than the full `swh.journal[testing]`. It
also avoids importing the whole kafka suite as soon as swh.journal is
installed.

conftest.py

+# Copyright (C) 2025 The Software Heritage developers
+# See the AUTHORS file at the top-level directory of this distribution
+# License: GNU General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+
+pytest_plugins = ["swh.journal.pytest_plugin"]

pyproject.toml

@@ -23,10 +23,8 @@ include = ["swh.*"]
 dependencies = {file = ["requirements.txt", "requirements-swh.txt"]}
 
 [tool.setuptools.dynamic.optional-dependencies]
-testing = {file = ["requirements.txt", "requirements-swh.txt", "requirements-test.txt"]}
-
-[project.entry-points.pytest11]
-"pytest_swh_journal" = "swh.journal.pytest_plugin"
+testing = {file = ["requirements-test.txt", "requirements-pytest.txt"]}
+pytest = {file = ["requirements-pytest.txt"]}
 
 [project.urls]
 "Homepage" = "https://gitlab.softwareheritage.org/swh/devel/swh-journal"

requirements-pytest.txt

+# Requirements for swh.journal.pytest_plugin
+attrs
+confluent-kafka
+pytest
+swh-model >= 6.14.0