Provide timezone information in datetime.fromtimestamp (#11468)

Python 3.12 has deprecated ``datetime.utcfromtimestamp``.
Resolve the warning by working out ``tzdelta`` via explicit timezone calculations.

Co-authored-by: Adam Turner <[email protected]>

Author: henryiii

sphinx/builders/gettext.py

@@ -4,7 +4,7 @@
 
 from codecs import open
 from collections import defaultdict
-from datetime import datetime, timedelta, tzinfo
+from datetime import datetime, timedelta, timezone, tzinfo
 from os import getenv, path, walk
 from time import time
 from typing import Any, Generator, Iterable
@@ -163,8 +163,10 @@ def write_doc(self, docname: str, doctree: nodes.document) -> None:
 
 # determine tzoffset once to remain unaffected by DST change during build
 timestamp = time()
-tzdelta = datetime.fromtimestamp(timestamp) - \
-    datetime.utcfromtimestamp(timestamp)
+local_time = datetime.fromtimestamp(timestamp)
+utc_time = datetime.fromtimestamp(timestamp, tz=timezone.utc)
+tzdelta = local_time - utc_time.replace(tzinfo=None)
+
 # set timestamp from SOURCE_DATE_EPOCH if set
 # see https://reproducible-builds.org/specs/source-date-epoch/
 source_date_epoch = getenv('SOURCE_DATE_EPOCH')