Expose niced_processes option through python API rather than package extra

[emlys]

@phargogh said on slack:

I can’t honestly remember why we didn’t just require psutil. psutil has been available on windows from PyPI at least as long as taskgraph has, so I doubt it’s an installation issue.
But also, the right thing to do for taskgraph should be to allow the user to define whether they want processes to be nice’d instead of changing behavior based on the available imports. So instead, I’d suggest that we require psutil at install time and then maybe allow the user to define (probably at Graph creation) whether they want to assign a process priority. If not, the priority should be inherited from the spawning process.