'Test that slices cannot be added as a pipeline column.'

def test_adding_slice_column(self):

    my_asset = self.asset_finder.retrieve_asset(self.sids[0])
    open_slice = OpenPrice()[my_asset]
    with self.assertRaises(UnsupportedPipelineOutput):
        Pipeline(columns={'open_slice': open_slice})
    pipe = Pipeline(columns={})
    with self.assertRaises(UnsupportedPipelineOutput):
        pipe.add(open_slice, 'open_slice')
