Enable basic type checking in demo platforms (#66212)

* Adjust type hints in demo platforms

* Adjust mypy config

* Adjust name

homeassistant/components/demo/number.py

@@ -105,13 +105,10 @@ class DemoNumber(NumberEntity):
         if step is not None:
             self._attr_step = step
 
-    @property
-    def device_info(self) -> DeviceInfo:
-        """Return device info."""
-        return DeviceInfo(
+        self._attr_device_info = DeviceInfo(
             identifiers={
                 # Serial numbers are unique identifiers within a specific domain
-                (DOMAIN, self.unique_id)
+                (DOMAIN, unique_id)
             },
             name=self.name,
         )