gh-103857: Document utcnow and utcfromtimestamp deprecations in What's New (#104542)

Co-authored-by: Paul Ganssle <[email protected]>

Author: hugovk

Doc/whatsnew/3.12.rst

@@ -774,6 +774,16 @@ Deprecated
   ``int``, convert to int explicitly with ``~int(x)``. (Contributed by Tim Hoffmann
   in :gh:`103487`.)
 
+* :class:`datetime.datetime`'s
+  :meth:`~datetime.datetime.utcnow` and
+  :meth:`~datetime.datetime.utcfromtimestamp` are deprecated and will be
+  removed in a future version. Instead, use timezone-aware objects to represent
+  datetimes in UTC: respectively, call
+  :meth:`~datetime.datetime.now` and
+  :meth:`~datetime.datetime.fromtimestamp`  with the *tz* parameter set to
+  :attr:`datetime.UTC`.
+  (Contributed by Paul Ganssle in :gh:`103857`.)
+
 Pending Removal in Python 3.13
 ------------------------------
 

Modules/_datetimemodule.c

@@ -5190,7 +5190,7 @@ datetime_utcfromtimestamp(PyObject *cls, PyObject *args)
     if (PyErr_WarnEx(PyExc_DeprecationWarning,
         "datetime.utcfromtimestamp() is deprecated and scheduled for removal "
         "in a future version. Use timezone-aware objects to represent "
-        "datetimes in UTC: datetime.now(datetime.UTC).", 1))
+        "datetimes in UTC: datetime.fromtimestamp(timestamp, datetime.UTC).", 1))
     {
         return NULL;
     }