`utcfromtimestamp` deprecated

```
/home/prod/secretlounge-ng/util/blacklist.py:130: DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).
  last_update = datetime.utcfromtimestamp(0.0)
```