use psutil.cpu_affinity() instead of os.cpu_count()

[altendky]

Purpose:

Hand off handling of various cases to psutil. Similar to #6131.

https://psutil.readthedocs.io/en/latest/#psutil.cpu_count

Note that psutil.cpu_count() may not necessarily be equivalent to the actual number of CPUs the current process can use. That can vary in case process CPU affinity has been changed, Linux cgroups are being used or (in case of Windows) on systems using processor groups or having more than 64 CPUs. The number of usable CPUs can be obtained with:

>>> len(psutil.Process().cpu_affinity())
1

Current Behavior:

New Behavior:

Testing Notes:

[github-actions]

File	Coverage	Missing Lines
chia/plotting/check_plots.py	0.0%	lines 29, 60
chia/util/beta_metrics.py	50.0%	lines 27

Total	Missing	Coverage
18 lines	3 lines	83%

[emlowe]

coverage diff exemption