Fix test_loop_auto for Python 3.14

[musicinmybrain]

Summary

In Python 3.14, asyncio.events.BaseDefaultEventLoopPolicy, which was always a private/undocumented API, was renamed with a leading underscore. See python/cpython#131148.

This PR adapts test_loop_auto, which contains a type check against this undocumented type, to support either name depending on the interpreter version, fixing failures like the following on Python 3.14:

________________________________ test_loop_auto ________________________________

    def test_loop_auto():
        auto_loop_setup()
        policy = asyncio.get_event_loop_policy()
>       assert isinstance(policy, asyncio.events.BaseDefaultEventLoopPolicy)
E       AttributeError: module 'asyncio.events' has no attribute 'BaseDefaultEventLoopPolicy'

tests/test_auto_detection.py:38: AttributeError

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change. (The change was to a test, so no new test was required.)
• I've updated the documentation accordingly. (No documentation changes were needed.)