TYP: annotate index/columns as Index

[jbrockmendel]

Series defined index and DataFrame defines index and columns as AxisProperty objects, which are property-like. These are not annotated as giving Index objects when accessed. I found in #51813 then when just adding index: Index to a mixin it caused 400ish mypy complaints elsewhere. The same thing happens if i add index: Index in NDFrame.

So it'd be nice if we can find a way to type AxisProperty.__get__ as returning an Index.

[jbrockmendel]

OK looks like AxisProperty.__get__ is already annotated. Not sure there is more we can do here. Closing.