This reformats our AST to match that introduced upstream by
 https://bugs.python.org/issue34822 and adjusts the conversion from
 older Python versions' AST to match.

ExtendedSliceAst is removed, in favour of a TupleAst containing
 SliceAsts and/or other expressions.

IndexAst is removed, instead non-slice index expressions are direct
 children of SubscriptAst.

SliceAst and EllipsisAst are now subclasses of ExpressionAst. In the
 latter case I don't understand how it wasn't broken before.

---

The stdlib docfiles are not updated.
 There aren't many changes to builtins in 3.9, but we do need a system to
 re-generate these - perhaps based on Typeshed.

In CPython 3.8 and 3.9 there are several changes to the parser that
 could be useful to us -- particularly end_lineno and end_col_offset.
 This patch doesn't attempt to take advantage of them.

BUG: 419290