Install scapy module:

apt install python-scapy

Create monitoring interface to capture 802.11 packets:

iw wlan0 interface add mon0 type monitor
ifconfig mon0 up

Run the script:

from scapy.all import *

def PacketHandler(pkt) :
  if pkt.haslayer(Dot11) :
    if pkt.type == 0 and pkt.subtype == 8 :
      if pkt.haslayer(Dot11Beacon) or pkt.haslayer(Dot11ProbeResp):
        try:
            extra = pkt.notdecoded
            rssi = -(256-ord(extra[-4:-3]))
        except:
            rssi = -100
        print "WiFi signal strength:", rssi, "dBm of", pkt.addr2, pkt.info

sniff(iface="mon0", prn = PacketHandler)