Invalidate relay cache after changing config.

Without this we don't see new relays after we add them.

pnpdevice/relays.py

@@ -175,6 +175,7 @@ class Manager:
 			"name": name,
 		})
 		_write_config(self.configpath, self.config)
+		self._relays = []
 
 	def relay_set(self, relay: Relay, state: bool) -> None:
 		if not relay.pin.chip.startswith("fake"):