Use timezone-aware objects to represent datetimes in UTC

[ludeeus]

With python 3.12 we start to see deprecation warnings for these:

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).

DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC)

As datetime.UTC does not exist in older versions, datetime.timezone.utc has been used instead.