TYP: Typing improvements for Index
Adds type hints to following:

- Add axis to Index.repeat
- Add sort to Index.union
- Add sort to Index.difference
- Add sort and result_name to Index.symmetric_difference
- Add level to Index.isin
- Add start, end, step to Index.slice_locs
- Add loc to Index.delete
- Add axis to Index.argmin
- Add axis to Index.argmax
- Add axis to Index.min
- Add axis to Index.max
- Add dtype to Index.astype

cc @phofl 