bpo-40334: Improve various PEG-Parser related stuff (GH-19669)

The changes in this commit are all related to @vstinner's original review comments of the initial PEP 617 implementation PR.
diff --git a/Parser/pegen/peg_api.c b/Parser/pegen/peg_api.c
index 7c6903c..c42aa68 100644
--- a/Parser/pegen/peg_api.c
+++ b/Parser/pegen/peg_api.c
@@ -1,4 +1,4 @@
-#include <pegen_interface.h>
+#include "pegen_interface.h"
 
 #include "../tokenizer.h"
 #include "pegen.h"