See https://github.com/tgalal/python-axolotl-curve25519/pull/26

diff --git a/curve25519module.c b/curve25519module.c
index f4bd3d7..cf8bb45 100644
--- a/curve25519module.c
+++ b/curve25519module.c
@@ -158,7 +158,7 @@ curve25519_functions[] = {
         PyModuleDef_HEAD_INIT,
         "axolotl_curve25519",
         NULL,
-        NULL,
+        0,
         curve25519_functions,
     };