API: A tuple passed to DataFame.sort_index will be interpreted as the levels of
     the index, rather than requiring a list of tuple (GH4370)
DEPR: remove 'columns' kw from DataFrame.sort (prior deprecated circa 2012)
Merge pull request #6639 from jreback/sort_error

API: A tuple passed to DataFame.sort_index will be interpreted as the levels of the index (GH4370)