Discard first DS18B12 measurement on ESP

Looks a bit like it's in fahrenheit, but it is definitely messing up my
graphs!

mqtt_sensord/sensorlib/esp_sensors.py

@@ -18,11 +18,18 @@ class DS18B20(Sensor):
         self.o = onewire.OneWire(machine.Pin(self.pin))
         self.ds = ds18x20.DS18X20(self.o)
         self.sID = self.ds.scan()[0]
+        self._first_measurement = True
 
     def get_data(self):
         self.ds.convert_temp()
         time.sleep(0.1)
-        return {"temp": self.ds.read_temp(self.sID)}
+        val = self.ds.read_temp(self.sID)
+        if self._first_measurement:
+            if val > 70.0:
+                val = None
+            else:
+                self._first_measurement = False
+        return {"temp": val}
 
 
 @sensor