TypeError when calling ESPSPI_WiFiManager.signal_strength

[mikejc58]

In adafruit_esp32spi_wifimanager.py, ESPSPI_WiFiManager.signal_strength() accesses self.esp.rssi as a function, but it is a property.

return self.esp.rssi() should be return self.esp.rssi

[siddacious]

Fixed via #111