add DateTimeUTC

[Frosty2500]

See here for the adaption of last_update, here for DateTimeUTC

[s-heppner]

From a feeling, I'd say this check should be before the attribute self.last_update is written.
However, this whole thing seems a little hacky, maybe we should discuss this in our next meeting.

[s-heppner]

Decision of the dev team:

• Implement DateTimeUTC as child of datetime.datetime with setter method that checks that the constraint that the datetime is given in timezone UTC when setting.
• Remove the timezone check at BasicEventElement.last_update

[s-heppner]

I tried to solve this and I decided we should leave it as it is currently implemented.

The only place DateTimeUTC is used is BasicEventElement.last_update. There, we used datetime.datetime and had a check for the setter:

    @last_update.setter
    def last_update(self, last_update: Optional[datatypes.DateTime]) -> None:
        if last_update is not None and last_update.tzname() != "UTC":
            raise ValueError("last_update must be specified in UTC!")
        self._last_update: Optional[datatypes.DateTime] = last_update

This makes the check implemented here in the __init__ of BasicEventElement unnecessary and our implementation compatible with the spec.

As long as the usage of events in the SDK are not fully specified, I'd keep it that way.