[PATCH 1 of 6] utils: stop using datetime.utcfromtimestamp() deprecated in Python 3.12

[Mads Kiilerich]

# HG changeset patch
# User Mads Kiilerich <mads at kiilerich.com>
# Date 1687866710 -7200
#      Tue Jun 27 13:51:50 2023 +0200
# Branch stable
# Node ID ec46e9b39b2a6f5d0c59aa803e2eb5d9dfc44cc3
# Parent  2b0598121a71fa19c2174e4eee3400ec3a3b1c26
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...

diff --git a/hgext/convert/common.py b/hgext/convert/common.py
--- a/hgext/convert/common.py
+++ b/hgext/convert/common.py
@@ -567,8 +567,10 @@ class mapfile(dict):
 
 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
diff --git a/mercurial/utils/dateutil.py b/mercurial/utils/dateutil.py
--- a/mercurial/utils/dateutil.py
+++ b/mercurial/utils/dateutil.py
@@ -83,10 +83,14 @@ def makedate(timestamp=None):
         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