The parser doesn't need its own implementation of assert, and having its
own interfered with including Python.h.  Remove Python's assert.h.
diff --git a/Parser/pgen.c b/Parser/pgen.c
index 47c817f..55ce7a1 100644
--- a/Parser/pgen.c
+++ b/Parser/pgen.c
@@ -4,8 +4,8 @@
 
 /* For a description, see the comments at end of this file */
 
+#include "Python.h"
 #include "pgenheaders.h"
-#include "assert.h"
 #include "token.h"
 #include "node.h"
 #include "grammar.h"