bpo-21120: Exclude Python-ast.h, ast.h and asdl.h from the limited API (#14634)

The PyArena type is not part of the limited API, so these headers
shouldn't be part of it either.
diff --git a/Include/Python-ast.h b/Include/Python-ast.h
index 5fe4f2b..b9232b1 100644
--- a/Include/Python-ast.h
+++ b/Include/Python-ast.h
@@ -6,6 +6,7 @@
 extern "C" {
 #endif
 
+#ifndef Py_LIMITED_API
 #include "asdl.h"
 
 #undef Yield   /* undefine macro conflicting with <winbase.h> */
@@ -708,6 +709,7 @@
 PyObject* PyAST_mod2obj(mod_ty t);
 mod_ty PyAST_obj2mod(PyObject* ast, PyArena* arena, int mode);
 int PyAST_Check(PyObject* obj);
+#endif /* !Py_LIMITED_API */
 
 #ifdef __cplusplus
 }