CLN: Consistent and Annotated Return Type of _iterate_slices
General pre-cursor to getting block management out of groupby. This is also a pre-cursor to fixing #21668 but needs to be coupled with a few more changes as a follow up

On master calls to _iterate_slices look up by label, potentially yielding a DataFrame if there were duplicated columns. This takes the surprise out of that and simply returns a Tuple of label / series for each item along the axis

@jbrockmendel 