Feature request: support for psutil 6.0

[Andrew-S-Rosen]

psutil 6.0.0 just came out! It'd be great if we could update requirements.txt to support it so codes depending on psij-python can update.

[hategan]

Thank you @Andrew-S-Rosen for keeping an eye on this. It is appreciated.
I have added psutil 6.0 to the requirements file along with 5.9 in #473. Based on a quick local test, there do not appear to be any visible issues with it.