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/Python/bltinmodule.c b/Python/bltinmodule.c
index 1888335..ce3561e 100644
--- a/Python/bltinmodule.c
+++ b/Python/bltinmodule.c
@@ -816,12 +816,12 @@
     if (str == NULL)
         goto error;
 
-    int current_use_peg = PyInterpreterState_Get()->config.use_peg;
+    int current_use_peg = PyInterpreterState_Get()->config._use_peg_parser;
     if (flags & PyCF_TYPE_COMMENTS || feature_version >= 0) {
-        PyInterpreterState_Get()->config.use_peg = 0;
+        PyInterpreterState_Get()->config._use_peg_parser = 0;
     }
     result = Py_CompileStringObject(str, filename, start[compile_mode], &cf, optimize);
-    PyInterpreterState_Get()->config.use_peg = current_use_peg;
+    PyInterpreterState_Get()->config._use_peg_parser = current_use_peg;
     Py_XDECREF(source_copy);
     goto finally;