Initial support for CPython 3.9
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
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