cpu_affinity() may raise OSError(EINVAL) on Linux

[giampaolo]

https://travis-ci.org/giampaolo/psutil/jobs/280295317

======================================================================
ERROR: psutil.tests.test_process.TestProcess.test_cpu_affinity
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/giampaolo/psutil/psutil/tests/test_process.py", line 874, in test_cpu_affinity
    p.cpu_affinity([n])
  File "/home/travis/build/giampaolo/psutil/psutil/__init__.py", line 871, in cpu_affinity
    self._proc.cpu_affinity_set(list(set(cpus)))
  File "/home/travis/build/giampaolo/psutil/psutil/_pslinux.py", line 1365, in wrapper
    return fun(self, *args, **kwargs)
  File "/home/travis/build/giampaolo/psutil/psutil/_pslinux.py", line 1773, in cpu_affinity_set
    cext.proc_cpu_affinity_set(self.pid, cpus)
OSError: [Errno 22] Invalid argument

[giampaolo]

Closing as duplicate of #1126.