On Linux with an old kernel:

0:03:59 load avg: 5.97 [300/420/1] test_posix failed -- running: test_tools (1 min 11 sec), test_concurrent_futures (2 min 42 sec)
test test_posix failed -- Traceback (most recent call last):
  File "/usr/local/src/sage-config/local/src/cpython/Lib/test/test_posix.py", line 311, in test_preadv_flags
    self.assertEqual(posix.preadv(fd, buf, 3, os.RWF_HIPRI), 10)
OSError: [Errno 95] Operation not supported

The problem is obvious: it's testing a flag which is not supported by this kernel. The fact that the macro RWF_HIPRI is defined (which is a compile-time condition) does not imply that the kernel actually supports it (which is a run-time condition).