bpo-34822: Simplify AST for subscription. (GH-9605)

* Remove the slice type.
* Make Slice a kind of the expr type instead of the slice type.
* Replace ExtSlice(slices) with Tuple(slices, Load()).
* Replace Index(value) with a value itself.

All non-terminal nodes in AST for expressions are now of the expr type.
15 files changed