open_meteo: correct UTC timezone handling in hourly forecast

The `datetime` variable is timezone-naive. Using
`dt_util.as_utc(datetime)` incorrectly assigns the local timezone
instead of UTC, leading to incorrect conversions. To fix this,
directly assign the UTC timezone to the `datetime` variable before
conversion, ensuring `dt_util.as_utc()` does not overwrite it with the
local timezone.

homeassistant/components/open_meteo/weather.py

@@ -156,6 +156,8 @@ class OpenMeteoWeatherEntity(
 
         hourly = self.coordinator.data.hourly
         for index, datetime in enumerate(self.coordinator.data.hourly.time):
+            if datetime.tzinfo is None:
+                datetime = datetime.replace(tzinfo=dt_util.UTC)
             if dt_util.as_utc(datetime) < today:
                 continue