python: Add a missing Py_INCREF() in nxt_python_start()

On some Python 3.11 systems, 3.11.9 & 3.11.10, we were seeing a crash
triggered by Py_Finalize() in nxt_python_start() when running one of our
pytests, namely
test/test_python_factory.py::test_python_factory_invalid_callable_value

  2024/09/12 15:07:29 [alert] 5452#5452 factory "wsgi_invalid_callable" in module "wsgi" can not be called to fetch callable
  Fatal Python error: none_dealloc: deallocating None: bug likely caused by a refcount error in a C extension
  Python runtime state: finalizing (tstate=0x00007f560b88a718)

  Current thread 0x00007f560bde7ad0 (most recent call first):
    <no Python frame>
  2024/09/12 15:07:29 [alert] 5451#5451 app process 5452 exited on signal 6 (core dumped)

It seems this was caused by

  Py_XDECREF(nxt_py_stderr_flush);

in nxt_python_atexit().

We never explicitly INCREF() this variable (though there are plenty of
other cases of doing DECREF()'s with no previous INCREF()'s). However in
this case adding an INCREF() for this variable does fix the crash.

I'm not sure we really need to reference count this variable, it's only
used in nxt_python_print_exception(), but it's perhaps the safest course
of action.

Cc: Konstantin Pavlov <[email protected]>
Closes: #1413
Signed-off-by: Andrew Clayton <[email protected]>

src/python/nxt_python.c

@@ -258,6 +258,8 @@ nxt_python_start(nxt_task_t *task, nxt_process_data_t *data)
         goto fail;
     }
 
+    Py_INCREF(nxt_py_stderr_flush);
+
     if (nxt_slow_path(nxt_python_set_path(task, c->path) != NXT_OK)) {
         goto fail;
     }