utils: stop using datetime.utcfromtimestamp() deprecated in Python 3.12 stable

utils: stop using datetime.utcfromtimestamp() deprecated in Python 3.12

Python3.12 made tests fail with warnings:
  DeprecationWarning: datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.fromtimestamp(timestamp, datetime.UTC).

Computing the diff while in timestamp seconds seems to preserve to the original
intent from ae04af1ce78d.

It would be nice to have some doctest coverage of this, with the problematic
corner cases that has popped up over time...

--- a/hgext/convert/common.py	Thu Jul 06 16:04:36 2023 +0200
+++ b/hgext/convert/common.py	Tue Jun 27 13:51:50 2023 +0200
@@ -567,8 +567,10 @@
 
 def makedatetimestamp(t):
     """Like dateutil.makedate() but for time t instead of current time"""
-    delta = datetime.datetime.utcfromtimestamp(
+    tz = round(
         t
-    ) - datetime.datetime.fromtimestamp(t)
-    tz = delta.days * 86400 + delta.seconds
+        - datetime.datetime.fromtimestamp(t)
+        .replace(tzinfo=datetime.timezone.utc)
+        .timestamp()
+    )
     return t, tz

--- a/mercurial/utils/dateutil.py	Thu Jul 06 16:04:36 2023 +0200
+++ b/mercurial/utils/dateutil.py	Tue Jun 27 13:51:50 2023 +0200
@@ -83,10 +83,14 @@
         raise error.InputError(
             _(b"negative timestamp: %d") % timestamp, hint=hint
         )
-    delta = datetime.datetime.utcfromtimestamp(
+    tz = round(
         timestamp
-    ) - datetime.datetime.fromtimestamp(timestamp)
-    tz = delta.days * 86400 + delta.seconds
+        - datetime.datetime.fromtimestamp(
+            timestamp,
+        )
+        .replace(tzinfo=datetime.timezone.utc)
+        .timestamp()
+    )
     return timestamp, tz