Fix import on Python 3.14 after asyncio policy API changes

[emirmrkaljevic]

Fixes #160.

On Python 3.14, import trio_asyncio fails with AttributeError because
CPython renamed asyncio.events.BaseDefaultEventLoopPolicy to
_BaseDefaultEventLoopPolicy and removed asyncio.AbstractChildWatcher
(see also #152).

This PR resolves the policy and child-watcher base classes from whatever
the interpreter still provides, and uses the non-deprecated private
policy helpers on 3.14 when installing our loop policy at import time.
Older Python versions keep the previous behavior.

Note: with this change the import works on 3.14 and the new compat tests pass, but the full test suite on 3.14 still hangs in some aio_as_trio tests. That looks like a separate issue outside #160, so I left it out of this PR.

[emirmrkaljevic]

The failing jobs look unrelated to this change. The pypy-3.9-nightly jobs fail during setup because there's no nightly PyPy 3.9 build available, and on Ubuntu 3.11/3.12 the only failure is CPython's own SslProtoHandshakeTests::test_check_hostname_requires_server_hostname (unraisable ResourceWarning during cleanup). #165, which touches different files, hits exactly the same failures; see the analysis there. Would a review be possible when you have time? Happy to adjust anything.