Clean up deprecation warnings

[pabs3]

• Fix deprecation warning

[jwilk]

https://github.com/projecthamster/hamster/pull/774/checks failed:

ERROR: test_round_trip (tests.test_stuff.TestDBus)
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Traceback (most recent call last):
  File "/home/runner/work/hamster/hamster/tests/test_stuff.py", line 506, in test_round_trip
    return_fact = from_dbus_fact(dbus_fact)
  File "/home/runner/work/hamster/hamster/src/hamster/lib/dbus.py", line 98, in from_dbus_fact
    start_time=dt.datetime.fromtimestamp(dbus_fact[1], dt.UTC),
AttributeError: module 'hamster.lib.datetime' has no attribute 'UTC'