pythonPackages.selectors2: fix build by disabling failing test_above_fd_setsize

This fixes the following error in the `selectors2` test suite:

```
======================================================================
ERROR: test_above_fd_setsize (tests.test_selectors2.PollSelectorTestCase)
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Traceback (most recent call last):
  File "/private/var/folders/5h/b11m6fxj2tqgbxwz5bgjy42c0000gn/T/nix-build-python3.8-selectors2-2.0.1.drv-0/selectors2-2.0.1/tests/test_selectors2.py", line 530, in test_above_fd_setsize
    self.assertEqual(limit_nofile // 2, len(s.select()))
  File "/private/var/folders/5h/b11m6fxj2tqgbxwz5bgjy42c0000gn/T/nix-build-python3.8-selectors2-2.0.1.drv-0/selectors2-2.0.1/selectors2.py", line 402, in select
    fd_events = _syscall_wrapper(self._wrap_poll, True, timeout=timeout)
  File "/private/var/folders/5h/b11m6fxj2tqgbxwz5bgjy42c0000gn/T/nix-build-python3.8-selectors2-2.0.1.drv-0/selectors2-2.0.1/selectors2.py", line 662, in _syscall_wrapper
    return func(*args, **kwargs)
  File "/private/var/folders/5h/b11m6fxj2tqgbxwz5bgjy42c0000gn/T/nix-build-python3.8-selectors2-2.0.1.drv-0/selectors2-2.0.1/selectors2.py", line 397, in _wrap_poll
    result = self._poll.poll(timeout)
OSError: [Errno 22] Invalid argument

----------------------------------------------------------------------
```