python3Packages.psycopg: disable test_sched{,_error} on darwin

They are prone to race conditions.

```
_______________________________ test_sched_error _______________________________
caplog = <_pytest.logging.LogCaptureFixture object at 0x197218a90>
    @pytest.mark.slow
    def test_sched_error(caplog):
        caplog.set_level(logging.WARNING, logger="psycopg")
        s = Scheduler()
        t = Thread(target=s.run, daemon=True)
        t.start()
        results = []
        def worker(i):
            results.append((i, time()))
        def error():
            1 / 0
        t0 = time()
        s.enter(0.1, partial(worker, 1))
        s.enter(0.4, None)
        s.enter(0.3, partial(worker, 2))
        s.enter(0.2, error)
        t.join()
        t1 = time()
        assert t1 - t0 == pytest.approx(0.4, 0.1)
        assert len(results) == 2
        assert results[0][0] == 1
>       assert results[0][1] - t0 == pytest.approx(0.1, 0.1)
E       assert 0.11003494262695312 == 0.1 ± 1.0e-02
E         comparison failed
E         Obtained: 0.11003494262695312
E         Expected: 0.1 ± 1.0e-02
tests/pool/test_sched.py:93: AssertionError
```

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