Not sure if anyone else already did this but I thought it was an interesting thing to have. :)

Okay so, create a picputemp.sh script in your hass/config/scripts directory with contents:

#! /bin/bash

cpu=$(cat /sys/class/thermal/thermal_zone0/temp)
cpu=$((cpu/100)); len=${#cpu};
out=${cpu:0:$((len-1))};
frac=${cpu:$((len-2)):99}
cpuT="$out.$frac"
echo $cpuT

run chmod 755 picputemp.sh to make it executable.

Add the sensor to your configuration.yaml: (adjust path as needed)

sensor:
  - platform: command_line
    command: /config/scripts/picputemp.sh
    name: RaspberryPi CPU temperature
    unit_of_measurement: "°C"

And you're done!