Tests/FindPython: only use limited API

Newer Python versions (namely 3.13) do not support old compilers before
gcc 4.8 for atomics, which leads to compile errors in the test case.
Since the test actually does not use any of that API, restrict it to the
limited Python API which offers even more stability across versions.

Tests/FindPython/spam.c

@@ -1,4 +1,4 @@
-
+#define Py_LIMITED_API 3
 #include <Python.h>
 
 static PyObject* spam_system(PyObject* self, PyObject* args)