some differences with generic thermostat

[vindaalex]

I did some checks to the official generic thermostat and found some differences. Should these be included?

double _async_async: https://github.com/fabiannydegger/custom_components/blob/22a1572bcbd9327baab10e7520cc54bc8470939a/smart_thermostat/climate.py#L335

missing preset_none: https://github.com/fabiannydegger/custom_components/blob/22a1572bcbd9327baab10e7520cc54bc8470939a/smart_thermostat/climate.py#L310-L311

to:

        if self._away_temp:
            return [PRESET_NONE, PRESET_AWAY]
        return None

check for preset mode: https://github.com/fabiannydegger/custom_components/blob/22a1572bcbd9327baab10e7520cc54bc8470939a/smart_thermostat/climate.py#L464

to: elif preset_mode == PRESET_NONE and self._is_away: