Fix compatibility w/ older version

Older version of Py (3.8) doesn't have dateime.UTC.

libs/lib/xaal/lib/messages.py

@@ -342,8 +342,9 @@ def build_nonce(data: tuple) -> bytes:
 
 def build_timestamp() -> tuple:
     """Return array [seconds since epoch, microseconds since last seconds] Time = UTC+0000"""
-    epoch = datetime.datetime.fromtimestamp(0, datetime.UTC)
-    timestamp = datetime.datetime.now(datetime.UTC) - epoch
+    utc = datetime.timezone.utc
+    epoch = datetime.datetime.fromtimestamp(0, utc)
+    timestamp = datetime.datetime.now(utc) - epoch
     return (int(timestamp.total_seconds()), int(timestamp.microseconds))