fix: Use asyncio.run for full Python 3.14 compatibility

[miaow2]

Follow-up to #123, which fixed only one of three broken call sites and used an API that 3.14 itself deprecates.

Background

Python 3.14 turns the long-deprecated implicit-loop creation in asyncio.get_event_loop() into a hard RuntimeError. Verified on CPython 3.14.0a6:

asyncio.get_event_loop()      -> RuntimeError: There is no current event loop in thread 'MainThread'.
asyncio.set_event_loop(loop)  -> DeprecationWarning: 'asyncio.set_event_loop' is deprecated
                                 and slated for removal in Python 3.16

What #123 left broken

• Configurator.push_configs() — a separate job entry point (jobs.py:35, enqueued from views/configuration.py and api/views.py) that never calls collect_diffs() first. Under the RQ work-horse it is the first asyncio call in a fresh process, so applying a ConfigurationRequest still died with RuntimeError before touching a device.
• ConfigDiffBase.get_actual_configs() — the connect-to-device branch taken whenever data_source and custom_field are both empty, i.e. the plugin's headline compliance script.
• asyncio.set_event_loop() — deprecated in 3.14, removed in 3.16, so the guard buys one release and breaks again.
• The guard covered the wrong statement. try wrapped only get_event_loop(). A thread that already has a closed loop set (asgiref's AsyncToSync, or any set_event_loop(l) + l.close()) gets that loop back without raising, and run_until_complete() then fails with RuntimeError: Event loop is closed. The newly created loop was also never closed, leaking an epoll fd plus self-pipe per thread.

This PR

All three call sites use asyncio.run(), which is correct on 3.10 through 3.16+, needs no try/except, and closes the loop and shuts down async generators. asyncio.gather() in get_actual_configs() is wrapped in a new _get_actual_configs() coroutine, since gather() also requires a running loop.

Dropped the blanket -p no:warnings from pytest addopts — it hid this exact DeprecationWarning for the two releases (3.12, 3.13) before it became an error, and would have hidden the new set_event_loop one the same way. Added 3.13/3.14 to the classifiers and the lint matrix.

The compliance/secrets.py line is unrelated: a pre-existing over-indented pass that ruff format normalizes. CI runs ruff format ., which rewrites files rather than failing, so it was never caught. Happy to split it out.

ruff format and ruff check pass; no get_event_loop/new_event_loop/set_event_loop/run_until_complete remains under netbox_config_diff/.

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