ENH: test_perf.py - use psutil to set affinity (if absent functionality - then affinity module)

original affinity module is not commonly installed, has known issues,
and was not updated since 2006.  psutil is already used in parts of
pandas, so it would be logical to use it instead by default.
minor: some trailing spaces and a pylint "pragma" to stop complaining about Series._ix defined elsewhere
Merge pull request #4296 from neurodebian/master

Use psutil to set process affinity (test_perf.py) + minor pylint-friendliness for ._ix