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GH-38626: [Python] Fix segfault when PyArrow is imported at shutdown (#38637)


### Rationale for this change

Some C++ destructors may be called after the Python interpreter has ceased to exist.
If such a destructor tries to call back in the Python interpreter, for example by calling `Py_DECREF`, we get a crash.

### What changes are included in this PR?

Protect `OwnedRef` and `OwneRefNoGIL` destructors against decref'ing a Python object after Python finalization.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #38626

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>