Now that the default parser is the new PEG parser, ast.parse uses it, which means that we don't actually test something in test_peg_parser. This commit introduces a new keyword argument (`oldparser`) for `_peg_parser.parse_string` for specifying that a string needs to be parsed with the old parser. This keyword argument is used in the tests to actually compare the ASTs the new parser generates with those generated by the old parser.
* Rename PyConfig.use_peg to _use_peg_parser
* Document PyConfig._use_peg_parser and mark it a deprecated
* Mark -X oldparser option and PYTHONOLDPARSER env var as deprecated
in the documentation.
* Add use_old_parser() and skip_if_new_parser() to test.support
* Remove sys.flags.use_peg: use_old_parser() uses
_testinternalcapi.get_configs() instead.
* Enhance test_embed tests
* subprocess._args_from_interpreter_flags() copies -X oldparser
