Enable long-term statistics

Set state_class to 'measurement' so long-term statistics are generated

Testing:
I made this on my local copy and noticed this sensor was now an option in the statistics card

Author: prestomation

custom_components/redfin/sensor.py

@@ -74,6 +74,11 @@ def state(self):
         except ValueError:
             return None
 
+    @property
+    def state_class(self):
+        # set state_class to 'measurement' so long-term statistics are generated
+        return STATE_CLASS_MEASUREMENT
+
     @property
     def extra_state_attributes(self):
         """Return the state attributes."""