gh-91321: Add _testcppext C++ extension (#32175)

Build a basic C++ test extension to check that the Python C API is
compatible with C++ and does not emit C++ compiler warnings.

* Add Modules/_testcppext.cpp: C++ extension
* Add Lib/test/test_cppext.py: test building the C++ extension.

Lib/test/_testcppext.cpp

@@ -0,0 +1,62 @@
+// gh-91321: Very basic C++ test extension to check that the Python C API is
+// compatible with C++ and does not emit C++ compiler warnings.
+
+#include "Python.h"
+
+PyDoc_STRVAR(_testcppext_add_doc,
+"add(x, y)\n"
+"\n"
+"Return the sum of two integers: x + y.");
+
+static PyObject *
+_testcppext_add(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    long i, j;
+    if (!PyArg_ParseTuple(args, "ll:foo", &i, &j)) {
+        return nullptr;
+    }
+    long res = i + j;
+    return PyLong_FromLong(res);
+}
+
+
+static PyMethodDef _testcppext_methods[] = {
+    {"add", _testcppext_add, METH_VARARGS, _testcppext_add_doc},
+    {nullptr, nullptr, 0, nullptr}  /* sentinel */
+};
+
+
+static int
+_testcppext_exec(PyObject *module)
+{
+    if (PyModule_AddIntMacro(module, __cplusplus) < 0) {
+        return -1;
+    }
+    return 0;
+}
+
+static PyModuleDef_Slot _testcppext_slots[] = {
+    {Py_mod_exec, reinterpret_cast<void*>(_testcppext_exec)},
+    {0, NULL}
+};
+
+
+PyDoc_STRVAR(_testcppext_doc, "C++ test extension.");
+
+static struct PyModuleDef _testcppext_module = {
+    PyModuleDef_HEAD_INIT,  // m_base
+    "_testcppext",  // m_name
+    _testcppext_doc,  // m_doc
+    0,  // m_size
+    _testcppext_methods,  // m_methods
+    _testcppext_slots,  // m_slots
+    NULL,  // m_traverse
+    NULL,  // m_clear
+    nullptr,  // m_free
+};
+
+PyMODINIT_FUNC
+PyInit__testcppext(void)
+{
+    return PyModuleDef_Init(&_testcppext_module);
+}