gh-141563: Fix test_cext on Windows (#144677)

The 'module' argument is now always needed to call the test_datetime
method.

Author: vstinner

Lib/test/test_cext/extension.c

@@ -74,13 +74,7 @@ static PyMethodDef _testcext_methods[] = {
 
 
 static int
-_testcext_exec(
-#ifdef __STDC_VERSION__
-    PyObject *module
-#else
-    PyObject *Py_UNUSED(module)
-#endif
-    )
+_testcext_exec(PyObject *module)
 {
     PyObject *result;