bpo-40334: Rename PyConfig.use_peg to _use_peg_parser (GH-19670)

* 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

1 files changed, 2 insertions, 1 deletions

diff --git a/Lib/test/test_peg_parser.py b/Lib/test/test_peg_parser.py
index 5aa6c0d8f4..ea4afa6e17 100644
--- a/Lib/test/test_peg_parser.py
+++ b/Lib/test/test_peg_parser.py
@@ -6,6 +6,7 @@ import unittest
 from pathlib import PurePath
 from typing import Any, Union, Iterable, Tuple
 from textwrap import dedent
+from test import support
 
 
 TEST_CASES = [
@@ -720,7 +721,7 @@ class ASTGenerationTest(unittest.TestCase):
                 f"Actual error message does not match expexted for {source}"
             )
 
-    @unittest.skipIf(sys.flags.use_peg, "This tests nothing for now, since compile uses pegen as well")
+    @support.skip_if_new_parser("This tests nothing for now, since compile uses pegen as well")
     @unittest.expectedFailure
     def test_correct_but_known_to_fail_ast_generation_on_source_files(self) -> None:
         for source in GOOD_BUT_FAIL_SOURCES: