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_eof.py b/Lib/test/test_eof.py
index bb1300c..f806578 100644
--- a/Lib/test/test_eof.py
+++ b/Lib/test/test_eof.py
@@ -26,7 +26,7 @@
         else:
             raise support.TestFailed
 
-    @unittest.skipIf(sys.flags.use_peg, "TODO for PEG -- fails with new parser")
+    @support.skip_if_new_parser("TODO for PEG -- fails with new parser")
     def test_line_continuation_EOF(self):
         """A continuation at the end of input must be an error; bpo2180."""
         expect = 'unexpected EOF while parsing (<string>, line 1)'