Fix another Python 3.12 deprecation warning

Another usage of utcfromtimestamp, this time in the obstruction map
script.

Author: sparky8512

dish_obstruction_map.py

@@ -31,7 +31,7 @@
 """
 
 import argparse
-from datetime import datetime
+from datetime import datetime, timezone
 import logging
 import os
 import png
@@ -92,8 +92,9 @@ def pixel_bytes(row):
     else:
         now = int(time.time())
         filename = opts.filename.replace("%u", str(now))
-        filename = filename.replace("%d",
-                                    datetime.utcfromtimestamp(now).strftime("%Y_%m_%d_%H_%M_%S"))
+        filename = filename.replace(
+            "%d",
+            datetime.fromtimestamp(now, timezone.utc).strftime("%Y_%m_%d_%H_%M_%S"))
         filename = filename.replace("%s", str(opts.sequence))
         out_file = open(filename, "wb")
     if not snr_data or not snr_data[0]: