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_codeop.py b/Lib/test/test_codeop.py
index f1d74b1..8c3e447 100644
--- a/Lib/test/test_codeop.py
+++ b/Lib/test/test_codeop.py
@@ -4,12 +4,12 @@
 """
 import sys
 import unittest
-from test.support import is_jython
+from test import support
 
 from codeop import compile_command, PyCF_DONT_IMPLY_DEDENT
 import io
 
-if is_jython:
+if support.is_jython:
 
     def unify_callables(d):
         for n,v in d.items():
@@ -21,7 +21,7 @@
 
     def assertValid(self, str, symbol='single'):
         '''succeed iff str is a valid piece of code'''
-        if is_jython:
+        if support.is_jython:
             code = compile_command(str, "<input>", symbol)
             self.assertTrue(code)
             if symbol == "single":
@@ -60,7 +60,7 @@
         av = self.assertValid
 
         # special case
-        if not is_jython:
+        if not support.is_jython:
             self.assertEqual(compile_command(""),
                              compile("pass", "<input>", 'single',
                                      PyCF_DONT_IMPLY_DEDENT))
@@ -122,7 +122,7 @@
         av("def f():\n pass\n#foo\n")
         av("@a.b.c\ndef f():\n pass\n")
 
-    @unittest.skipIf(sys.flags.use_peg, "Pegen does not support PyCF_DONT_INPLY_DEDENT yet")
+    @support.skip_if_new_parser("Pegen does not support PyCF_DONT_INPLY_DEDENT yet")
     def test_incomplete(self):
         ai = self.assertIncomplete