test_when_timeout_smaller_second is flaky

[derlih]

🐞 Describe the bug
Sometimes test_when_timeout_smaller_second fails

💡 To Reproduce
Run this test multiple times

pip install pytest-repeat
pytest -q tests/test_helpers.py::test_when_timeout_smaller_second --count 1000 -x

💡 Expected behavior
Test shouldn't fail

📋 Logs/tracebacks

loop = <_UnixSelectorEventLoop running=False closed=False debug=False>

    def test_when_timeout_smaller_second(loop) -> None:
        timeout = 0.1
        timer = loop.time() + timeout
    
        handle = helpers.TimeoutHandle(loop, timeout)
        when = handle.start()._when
        handle.close()
    
        assert isinstance(when, float)
>       assert f"{when:.3f}" == f"{timer:.3f}"
E       AssertionError: assert '151331.472' == '151331.471'
E         - 151331.471
E         ?          ^
E         + 151331.472
E         ?          ^

📋 Your version of the Python

$ python --version
Python 3.8.5

📋 Your version of the aiohttp/yarl/multidict distributions

$ python -m pip show aiohttp
4.0.0a1

$ python -m pip show multidict
5.0.0

$ python -m pip show yarl
1.6.1

[asvetlov]

Fixed by #5117