Python - how to check system load?

As suggested in this answer maybe using:

>>> import os
>>> os.getloadavg()
(0.66, 0.69, 0.58)

It's more what you're looking for since that's the server load, not just the cpu usage.

Try psutil (https://github.com/giampaolo/psutil):

import psutil
psutil.cpu_percent()

EDIT 2019/04/26: psutil now implements and emulates getloadavg() also on Windows:

 >>> psutil.getloadavg()
 (3.14, 3.89, 4.67)