Add and use percentage constant (#32094)

* Add and use percentage constant

* Fix pylint error and broken test

homeassistant/components/lacrosse/sensor.py

@@ -15,6 +15,7 @@ from homeassistant.const import (
     CONF_TYPE,
     EVENT_HOMEASSISTANT_STOP,
     TEMP_CELSIUS,
+    UNIT_PERCENTAGE,
 )
 from homeassistant.core import callback
 import homeassistant.helpers.config_validation as cv
@@ -193,7 +194,7 @@ class LaCrosseHumidity(LaCrosseSensor):
     @property
     def unit_of_measurement(self):
         """Return the unit of measurement."""
-        return "%"
+        return UNIT_PERCENTAGE
 
     @property
     def state(self):