COMPAT: 3.6.1 compat for change in PySlice_GetIndices_Ex
This doesn't actually matter to any tests except for some internal consistency ones.
Bonus is that it eliminates a warning :<

note that we aren't actually testing this (yet) on Travis as our 3.6 build uses conda-forge and 3.6.1 is not there as of yet. Its in defaults though (and shows up on appveyor build).