Use UTC timezone when handling keys in kasp test library

When working with key timestamps, ensure we correctly set the UTC
timezone in order for the tests to work consistently regardless of the
local time setting.

(cherry picked from commit f840deba33)

bin/tests/system/isctest/kasp.py

@@ -17,8 +17,7 @@ import subprocess
 import time
 from typing import Optional, Union
 
-from datetime import datetime
+from datetime import datetime, timedelta, timezone
-from datetime import timedelta
 
 import dns
 import isctest.log
@@ -54,7 +53,9 @@ class KeyTimingMetadata:
     def __init__(self, timestamp: str):
         if int(timestamp) <= 0:
             raise ValueError(f'invalid timing metadata value: "{timestamp}"')
-        self.value = datetime.strptime(timestamp, self.FORMAT)
+        self.value = datetime.strptime(timestamp, self.FORMAT).replace(
+            tzinfo=timezone.utc
+        )
 
     def __repr__(self):
         return self.value.strftime(self.FORMAT)
@@ -95,7 +96,7 @@ class KeyTimingMetadata:
     @staticmethod
     def now() -> "KeyTimingMetadata":
         result = KeyTimingMetadata.__new__(KeyTimingMetadata)
-        result.value = datetime.now()
+        result.value = datetime.now(timezone.utc)
         return result