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/test/test_traceback.py

@@ -655,7 +655,6 @@ class BaseExceptionReportingTests:
         self.assertIn('inner_raise() # Marker', blocks[2])
         self.check_zero_div(blocks[2])
 
-    @unittest.skipIf(support.use_old_parser(), "Pegen is arguably better here, so no need to fix this")
     def test_syntax_error_offset_at_eol(self):
         # See #10186.
         def e():