gh-161032: Convert BaseEventLoop._clock_resolution to asyncio.constants.CLOCK_RESOLUTION - #161033

BaseEventLoop.__init__ queried time.get_clock_info('monotonic').resolution for every loop and stored it in the private _clock_resolution attribute, which only _run_once reads. The value is process-wide and never changes, so compute it once as asyncio.constants.CLOCK_RESOLUTION.

TestLoop no longer overrides the resolution with 1e-9. test_asyncio passes with the real resolution and with a forced Windows-like value of 0.015625 s.

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• Issue: asyncio: compute the event loop's clock resolution once, in asyncio.constants #161032