bpo-40939: Remove the old parser (GH-20768)

This commit removes the old parser, the deprecated parser module, the old parser compatibility flags and environment variables and all associated support code and documentation.
diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
index 9048e89..9eb7ebe 100644
--- a/Lib/test/test_fstring.py
+++ b/Lib/test/test_fstring.py
@@ -12,7 +12,7 @@
 import types
 import decimal
 import unittest
-from test.support import temp_cwd, use_old_parser
+from test.support import temp_cwd
 from test.support.script_helper import assert_python_failure
 
 a_global = 'global variable'
@@ -1049,7 +1049,6 @@
                              r"f'{1000:j}'",
                             ])
 
-    @unittest.skipIf(use_old_parser(), "The old parser only supports <fstring> as the filename")
     def test_filename_in_syntaxerror(self):
         # see issue 38964
         with temp_cwd() as cwd: