bpo-40939: Rename PyPegen* functions to PyParser* (GH-21016)

Rename PyPegen* functions to PyParser*, so that we can remove the
old set of PyParser* functions that were using the old parser.

diff --git a/Include/Python.h b/Include/Python.h
index dcd0a57..57f71d4 100644
--- a/Include/Python.h
+++ b/Include/Python.h
@@ -141,6 +141,7 @@
 #include "modsupport.h"
 #include "compile.h"
 #include "pythonrun.h"
+#include "parser_interface.h"
 #include "pylifecycle.h"
 #include "ceval.h"
 #include "sysmodule.h"