Fix compiler warning on Windows

..\Modules\_testcapimodule.c(3320): warning C4098: 'matmulType_dealloc' : 'void' function returning a value

Modules/_testcapimodule.c

@@ -3317,7 +3317,7 @@ matmulType_imatmul(PyObject *self, PyObject *other)
 static void
 matmulType_dealloc(PyObject *self)
 {
-    return Py_TYPE(self)->tp_free(self);
+    Py_TYPE(self)->tp_free(self);
 }
 
 static PyNumberMethods matmulType_as_number = {