requirements-db-pytestplugin: Don't install pytest-postgresql 3.0+

3.0 breaks backwards compatibility, we should handle the upgrade in a
separate patch, for now this ensures the rest of the pipelines can keep
running.

requirements-db-pytestplugin.txt

 # requirements for swh.core.db.pytest_plugin
-pytest-postgresql
+pytest-postgresql < 3.0.0