python: Don't decrement a reference to a borrowed object

On some Python 3.11 systems, 3.11.9 & 3.11.10, we were seeing a crash
triggered by Py_Finalize() in nxt_python_atexit() 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)

This was due to

  obj = PyDict_GetItemString(PyModule_GetDict(module), callable);

in nxt_python_set_target() which returns a *borrowed* reference, then
due to the test meaning this is a `None` object we `goto fail` and call

  Py_DECREF(obj);

which then causes `Py_Finalize()` to blow up.

The simple fix is to just increment its reference count before the `goto
fail`.

Note: This problem only showed up under (the various versions of Python
we test on); 3.11.9 & 3.11.10. It doesn't show up under; 3.6, 3.7, 3.9,
3.10, 3.12

Cc: Konstantin Pavlov <thresh@nginx.com>
Closes: https://github.com/nginx/unit/issues/1413
Fixes: a9aa9e76d ("python: Support application factories")
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

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