Make tests discovery compatible with pytest 6
Make tests discovery compatible with pytest 6

The 'args' keyword for the pytest.Function.from_parent constructor was removed
with pytest 6. It was unused in pytest [1] and was always an empty tuple in
the sasmodels code.

[1] https://github.com/pytest-dev/pytest/pull/7226

SasView/sasmodels,SasView/sasmodels,SasView/sasmodels,SasView/sasmodels