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_embed.py b/Lib/test/test_embed.py
index 24ebc5c..0bdfae1 100644
--- a/Lib/test/test_embed.py
+++ b/Lib/test/test_embed.py
@@ -347,7 +347,7 @@
         'isolated': 0,
         'use_environment': 1,
         'dev_mode': 0,
-        'use_peg': 1,
+        '_use_peg_parser': 1,
 
         'install_signal_handlers': 1,
         'use_hash_seed': 0,
@@ -729,7 +729,7 @@
             'import_time': 1,
             'show_ref_count': 1,
             'malloc_stats': 1,
-            'use_peg': 0,
+            '_use_peg_parser': 0,
 
             'stdio_encoding': 'iso8859-1',
             'stdio_errors': 'replace',
@@ -792,6 +792,7 @@
             'user_site_directory': 0,
             'faulthandler': 1,
             'warnoptions': ['EnvVar'],
+            '_use_peg_parser': 0,
         }
         self.check_all_configs("test_init_compat_env", config, preconfig,
                                api=API_COMPAT)
@@ -819,6 +820,7 @@
             'user_site_directory': 0,
             'faulthandler': 1,
             'warnoptions': ['EnvVar'],
+            '_use_peg_parser': 0,
         }
         self.check_all_configs("test_init_python_env", config, preconfig,
                                api=API_PYTHON)