timer: use datetime.timezone.utc for python 3.10

Python 3.10 doesn't have datetime.UTC yet as Python 3.12 does

Signed-off-by: Kyr Shatskyy <[email protected]>

teuthology/timer.py

@@ -68,7 +68,7 @@ def get_datetime_string(self, time):
 
         :param time: Time in seconds; like from time.time()
         """
-        _datetime = datetime.datetime.fromtimestamp(time, datetime.UTC)
+        _datetime = datetime.datetime.fromtimestamp(time, datetime.timezone.utc)
         return datetime.datetime.strftime(
             _datetime,
             self.datetime_format,