Add support for fauxmo devices to be discovered by HomeAssistant

Makes two changes needed for HomeAssistant to discover and manage
fauxmo devices:

* HomeAssistant relies on pywemo for device discovery. The
  pywemo.discover_devices() function sends an ssdp probe of the string
  "urn:Belkin:service:basicevent:1". This change causes fauxmo to respond to
  that probe so that pywemo.discover_devices will find fauxmo devices.

* As mentioned in #88,
  HomeAssistant uses the serial number returned by a "real" Wemo to
  disambiguate devices, if more than one device shares an IP
  address. This changes causes fauxmo to respond to a setup message
  with the serial number, so that multiple fauxmo devices will be seen
  from HomeAssistant.

src/fauxmo/protocols.py

@@ -81,6 +81,7 @@ def handle_setup(self) -> None:
             "<manufacturer>Belkin International Inc.</manufacturer>"
             "<modelName>Emulated Socket</modelName>"
             "<modelNumber>3.1415</modelNumber>"
+            f"<serialNumber>{self.serial}</serialNumber>"
             f"<UDN>uuid:Socket-1_0-{self.serial}</UDN>"
             "<serviceList>"
             "<service>"
@@ -362,6 +363,7 @@ def datagram_received(
 
         discover_patterns = [
             "ST: urn:Belkin:device:**",
+            "ST: urn:Belkin:service:basicevent:1",
             "ST: upnp:rootdevice",
             "ST: ssdp:all",
         ]