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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