See https://github.com/clawpack/pyclaw/pull/667

--- pyclaw/src/pyclaw/limiters/weno/reconstruct.c.orig	2020-04-27 00:20:31.000000000 -0700
+++ pyclaw/src/pyclaw/limiters/weno/reconstruct.c	2021-04-15 07:06:02.000000000 -0700
@@ -5263,6 +5263,21 @@
 PyMODINIT_FUNC
 initreconstruct (void)
 {
+#if PY_MAJOR_VERSION >= 3
+  static struct PyModuleDef moduledef = {
+    PyModuleDef_HEAD_INIT,
+    "reconstruct",       /* m_name */
+    NULL,                /* m_doc */
+    -1,                  /* m_size */
+    reconstructmethods,  /* m_methods */
+    NULL,                /* m_reload */
+    NULL,                /* m_traverse */
+    NULL,                /* m_clear */
+    NULL                 /* m_free */
+  };
+  PyModule_Create(&moduledef);
+#else
   (void) Py_InitModule ("reconstruct", reconstructmethods);
+#endif
   import_array ();
 }