gh-141563: Don't test datetime.h with the limited C API (#144673)

Fix test_cext and test_cppext.

Author: vstinner

Lib/test/test_cext/extension.c

@@ -54,10 +54,13 @@ _testcext_add(PyObject *Py_UNUSED(module), PyObject *args)
 static PyObject *
 test_datetime(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
 {
+    // datetime.h is excluded from the limited C API
+#ifndef Py_LIMITED_API
     PyDateTime_IMPORT;
     if (PyErr_Occurred()) {
         return NULL;
     }
+#endif
 
     Py_RETURN_NONE;
 }

Lib/test/test_cppext/extension.cpp

@@ -232,10 +232,13 @@ test_virtual_object(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
 static PyObject *
 test_datetime(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
 {
+    // datetime.h is excluded from the limited C API
+#ifndef Py_LIMITED_API
     PyDateTime_IMPORT;
     if (PyErr_Occurred()) {
         return NULL;
     }
+#endif
 
     Py_RETURN_NONE;
 }