_platform.get_cpu_count: use psutil instead of os

Angelos Evripiotisrequested to merge
aevri/psutil_affinity into master

Be Windows-friendly by using the portable psutil.Process().cpu_affinity() instead of os.sched_getaffinity.

Note that unfortunately this still doesn't work on Mac, so we keep that platform-specific override.

Edited by Angelos Evripiotis

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