pythongh-88994: Change datetime.datetime.now to half-even rounding (python#134258)

Change `datetime.datetime.now` to half-even rounding
for consistency with `datetime.fromtimestamp`.

Author: jhohm

Misc/ACKS

@@ -795,6 +795,7 @@ Albert Hofkamp
 Chris Hogan
 Tomas Hoger
 Jonathan Hogg
+John Keith Hohm
 Vladyslav Hoi
 Kamilla Holanda
 Steve Holden

Misc/NEWS.d/next/Library/2025-05-19-18-12-42.gh-issue-88994.7avvVu.rst

@@ -0,0 +1,3 @@
+Change :func:`datetime.datetime.now` to half-even rounding for
+consistency with :func:`datetime.datetime.fromtimestamp`.  Patch by
+John Keith Hohm.

Modules/_datetimemodule.c

@@ -5551,8 +5551,9 @@ datetime_best_possible(PyObject *cls, TM_FUNC f, PyObject *tzinfo)
     time_t secs;
     int us;
 
-    if (_PyTime_AsTimevalTime_t(ts, &secs, &us, _PyTime_ROUND_FLOOR) < 0)
+    if (_PyTime_AsTimevalTime_t(ts, &secs, &us, _PyTime_ROUND_HALF_EVEN) < 0) {
         return NULL;
+    }
     assert(0 <= us && us <= 999999);
 
     return datetime_from_timet_and_us(cls, f, secs, us, tzinfo);