gh-158903: Avoid AttributeError in BaseEventLoop.__del__ and close() during premature teardown

[JohnnyWilson16]

An interrupted event loop initialization (e.g. keyboard interrupt or signal received before _make_self_pipe() or proactor setup completes) leaves attributes like _ssock, _csock, _selector, _proactor, and _signal_handlers uninitialized or partially assigned. When garbage collected, BaseEventLoop.__del__ calls self.close() -> _close_self_pipe(), unconditionally dereferencing self._ssock.fileno() and raising an AttributeError via sys.unraisablehook.

This patch guards internal attribute dereferences across BaseSelectorEventLoop, _UnixSelectorEventLoop, and BaseProactorEventLoop during close() and _close_self_pipe(), ensuring partially initialized loops tear down cleanly.

• Added regression test test_close_partially_initialized_loop in Lib/test/test_asyncio/test_base_events.py verifying clean close and zero unraisable exceptions under support.catch_unraisable_exception().
• Verified via ./python -m test test_asyncio (33/33 suites passed).
• Includes NEWS blurb in Misc/NEWS.d/next/Library/.

Fixes #158903.

• Issue: Race condition leads to AttributeError from BaseEventLoop.__del__ #158903

[python-cla-bot]

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[picnixz]

This adds un-necessary defensive guards for a very rare event. And I don't think it's necessary to have it. I'm closing it because it's premature to do so and I'll Kumar decide on whether he wants to accept the issue.