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.

Lib/subprocess.py

@@ -326,7 +326,7 @@ def _args_from_interpreter_flags():
     if dev_mode:
         args.extend(('-X', 'dev'))
     for opt in ('faulthandler', 'tracemalloc', 'importtime',
-                'showrefcount', 'utf8', 'oldparser'):
+                'showrefcount', 'utf8'):
         if opt in xoptions:
             value = xoptions[opt]
             if value is True: