gateway: Add "enable_telnet" to gateway (rytilahti#734)

* Add "enable_telnet" to gateway

Apperently you can enable the telnet service on xiaomi gateways to get root acces into the operating system of the gateway.
Have not tested it myself (do not (yet) want to mess with the operating system).
But everything is discussed in this HomeAssistant community post: https://community.home-assistant.io/t/xiaomi-mijia-smart-multi-mode-gateway-zndmwg03lm-support/159586/61

* catch error on gateway models not supproting enable_telnet

* fix isort

miio/gateway.py

@@ -10,7 +10,7 @@
 
 from .click_common import EnumType, command, format_output
 from .device import Device
-from .exceptions import DeviceException
+from .exceptions import DeviceError, DeviceException
 from .utils import brightness_and_color_to_int, int_to_brightness, int_to_rgb
 
 _LOGGER = logging.getLogger(__name__)
@@ -357,6 +357,18 @@ def set_developer_key(self, key):
 
         return self.send("set_lumi_dpf_aes_key", [key])
 
+    @command()
+    def enable_telnet(self):
+        """Enable root telnet acces to the operating system, use login "admin" or "app", no password."""
+        try:
+            return self.send("enable_telnet_service")
+        except DeviceError:
+            _LOGGER.error(
+                "Gateway model '%s' does not (yet) support enabling the telnet interface",
+                self.model,
+            )
+            return None
+
     @command()
     def timezone(self):
         """Get current timezone."""