transf: future proof __getitem__#382

james-d-mitchell wants to merge 2 commits intolibsemigroups:stable-1.3libsemigroups/libsemigroups_pybind11:stable-1.3from

james-d-mitchell:fix-transfjames-d-mitchell/libsemigroups_pybind11:fix-transfCopy head branch name to clipboard