Investigate dropping psutil in favour of reading device nodes

[Gadgetoid]

It looks like in many (all?) cases we can just attempt to read /sys/class/thermal/thermal_zone0/temp and /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq to get the current CPU temperature and CPU scaling frequency- avoiding the psutil dependency.

Building some functions into Fan SHIM to report these figures would seem sensible.

This is a continuation of - https://github.com/pimoroni/fanshim-python/issues/58 - in an effort to get compatibility beyond just Raspbian.

[Gadgetoid]

This should also fix: https://github.com/pimoroni/fanshim-python/issues/13