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
diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
index 45f55e1..7361d09 100644
--- a/Lib/test/test_traceback.py
+++ b/Lib/test/test_traceback.py
@@ -656,8 +656,7 @@
         self.assertIn('inner_raise() # Marker', blocks[2])
         self.check_zero_div(blocks[2])
 
-    @unittest.skipIf(sys.flags.use_peg,
-                     "Pegen is arguably better here, so no need to fix this")
+    @support.skip_if_new_parser("Pegen is arguably better here, so no need to fix this")
     def test_syntax_error_offset_at_eol(self):
         # See #10186.
         def e():