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_exceptions.py b/Lib/test/test_exceptions.py
index 196f31e..feae31b 100644
--- a/Lib/test/test_exceptions.py
+++ b/Lib/test/test_exceptions.py
@@ -251,9 +251,9 @@
         check('def f():\n  x, y: int', 2, 3)
         check('[*x for x in xs]', 1, 2)
         check('foo(x for x in range(10), 100)', 1, 5)
-        check('(yield i) = 2', 1, 1 if support.use_old_parser() else 2)
-        check('def f(*):\n  pass', 1, 7 if support.use_old_parser() else 8)
-        check('for 1 in []: pass', 1, 5 if support.use_old_parser() else 7)
+        check('(yield i) = 2', 1, 2)
+        check('def f(*):\n  pass', 1, 8)
+        check('for 1 in []: pass', 1, 7)
 
     @cpython_only
     def testSettingException(self):