Fix late-import paho (#81791)

fix: late-import MQTTMessage

`paho-mqtt` is not listed in main requirements and is imported early by `conftest`.

Import it late to avoid an ImportError.

Split out from #81678.

homeassistant/components/mqtt/client.py

@@ -15,7 +15,6 @@ import uuid
 
 import attr
 import certifi
-from paho.mqtt.client import MQTTMessage
 
 from homeassistant.config_entries import ConfigEntry
 from homeassistant.const import (
@@ -620,7 +619,7 @@ class MQTT:
             )
 
     def _mqtt_on_message(
-        self, _mqttc: mqtt.Client, _userdata: None, msg: MQTTMessage
+        self, _mqttc: mqtt.Client, _userdata: None, msg: mqtt.MQTTMessage
     ) -> None:
         """Message received callback."""
         self.hass.add_job(self._mqtt_handle_message, msg)
@@ -634,7 +633,7 @@ class MQTT:
         return subscriptions
 
     @callback
-    def _mqtt_handle_message(self, msg: MQTTMessage) -> None:
+    def _mqtt_handle_message(self, msg: mqtt.MQTTMessage) -> None:
         _LOGGER.debug(
             "Received%s message on %s: %s",
             " retained" if msg.retain else "",