pytest.helpers.remove_stale_minion_key not registered

[os-sengel]

This line in conftest.py contains an unresolved reference, pytest.helpers.remove_stale_minion_key: https://github.com/saltstack/pepper/blob/8ab18e8fb81c275e19c1345146116cf14b9f1223/tests/conftest.py#L247-L249

I think it's likely, that the conftest.py was once derived from the main project's one. However, the respective helper function was never used to test pepper, therefore this missing reference never raised an exception.