bpo-40688: Use the correct parser in the peg_generator scripts (GH-20235)

bpo-40688: Use the correct parser in the peg_generator scripts (GH-20235)

The scripts in `Tools/peg_generator/scripts` mostly assume that
`ast.parse` and `compile` use the old parser, since this was the
state of things, while we were developing them. They need to be
updated to always use the correct parser. `_peg_parser` is being
extended to support both parsing and compiling with both parsers.
(cherry picked from commit 9645930b5bc1833ef495891d22052d1ba65ab7ea)

Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>