bug: `test_knockknock_some_gil` is flaky on newer macs (0.4.2-post3)

[sarahec]

While building for nixpkgs, test_knockknock_some_gil fails on my speedy MacBook Pro (M3 Max) with:

___________________________ test_knockknock_some_gil ___________________________

    def test_knockknock_some_gil():
        knocker = _run(some_gil)
    
        try:
            # usually ~0.75ish, but ~0.4 on mac
>           assert 0.2 < knocker.contention_metric < 0.9
E           assert 0.2 < 0.10452888906002045
E            +  where 0.10452888906002045 = <builtins.KnockKnock object at 0x1094a51b0>.contention_metric

tests/test_knockknock.py:80: AssertionError
----------------------------- Captured stdout call -----------------------------
Metric: 0.07699864357709885
----------------------------- Captured stdout call -----------------------------
Metric: 0.0730428621172905
----------------------------- Captured stdout call -----------------------------
Metric: 0.0822787880897522
----------------------------- Captured stdout call -----------------------------
Metric: 0.10452888906002045