Note: black cannot parse `return *active_fixture_argnames,
*self.argnames` yet (fixed in master, psf/black#1121).

Tested manually using:
```python
@pytest.fixture(scope="session")
def xdist_suffix(request):
    print("\nxdist_suffix")
    suffixes.append("xdist")

@pytest.fixture(scope="session")
def parallel_suffix(tox_suffix, xdist_suffix):
    pass

def test_suffix(parallel_suffix):
    assert suffixes == ["tox", "xdist"]
```

When using a set there the order is not deterministic, i.e. the test is
flaky.

…er finalizer to a dependee fixture once

… add ther finalizer to a dependee fixture once