RHEL 7: python: Check for _Py_IsFinalizing.

This symbol is not present in Python 2.7 or 3.6.  It's not really
necessary to call this, it just avoids a crash in certain corner cases
when the interpreter is shutting down.  So make the call conditional
on the function existing.

Fixes commit e6f9e0b0f6.

python/handle.c

@@ -121,8 +121,10 @@ guestfs_int_py_event_callback_wrapper (guestfs_h *g,
   PyObject *py_r;
   int threads_initialized = PyEval_ThreadsInitialized ();
 
+#ifdef HAVE_PY_ISFINALIZING
   if (_Py_IsFinalizing ())
     return;
+#endif
 
   if (threads_initialized)
     py_save = PyGILState_Ensure ();