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_fstring.py b/Lib/test/test_fstring.py
index 802b083..8cafbe8 100644
--- a/Lib/test/test_fstring.py
+++ b/Lib/test/test_fstring.py
@@ -12,6 +12,7 @@
 import decimal
 import sys
 import unittest
+from test import support
 
 a_global = 'global variable'
 
@@ -206,7 +207,7 @@
         call = binop.right.values[1].value
         self.assertEqual(type(call), ast.Call)
         self.assertEqual(call.lineno, 3)
-        if not sys.flags.use_peg:
+        if support.use_old_parser():
             self.assertEqual(call.col_offset, 11)
 
     def test_ast_line_numbers_duplicate_expression(self):