_dependee_fixture_argnames: return tuple

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.

src/_pytest/fixtures.py

@@ -962,7 +962,7 @@ def _dependee_fixture_argnames(self, request):
 
         stack_slice_index = min([current_fix_index, *parent_fixture_indexes])
         active_fixture_argnames = all_fix_names[:stack_slice_index]
-        return {*active_fixture_argnames, *self.argnames}
+        return tuple(active_fixture_argnames) + self.argnames
 
     def cache_key(self, request):
         return request.param_index if not hasattr(request, "param") else request.param