tests: replace deprecated datetime.utcfromtimestamp

datetime.utcfromtimestamp() is deprecated for removal. Use the
timezone-aware datetime.fromtimestamp(ts, timezone.utc), which produces
the identical UTC-aware value the assertion compares against.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: PetrHeinz

tests/test_frame.py

@@ -17,7 +17,7 @@ def test_create_frame_happy_path(self):
         self.assertTrue(frame['dt'].endswith("+00:00"))
 
         # Sent date matches log record date
-        date_ref = datetime.datetime.utcfromtimestamp(log_record.created).replace(tzinfo=datetime.timezone.utc)
+        date_ref = datetime.datetime.fromtimestamp(log_record.created, datetime.timezone.utc)
         date_sent = datetime.datetime.fromisoformat(frame['dt'])
         self.assertEqual(date_ref, date_sent)