Issue #23517: fromtimestamp() and utcfromtimestamp() methods of

datetime.datetime now round microseconds to nearest with ties going to nearest
even integer (ROUND_HALF_EVEN), as round(float), instead of rounding towards
-Infinity (ROUND_FLOOR).

pytime API: replace _PyTime_ROUND_HALF_UP with _PyTime_ROUND_HALF_EVEN. Fix
also _PyTime_Divide() for negative numbers.

_PyTime_AsTimeval_impl() now reuses _PyTime_Divide() instead of reimplementing
rounding modes.

Modules/_datetimemodule.c

@@ -4103,7 +4103,7 @@ datetime_from_timestamp(PyObject *cls, TM_FUNC f, PyObject *timestamp,
     long us;
 
     if (_PyTime_ObjectToTimeval(timestamp,
-                                &timet, &us, _PyTime_ROUND_HALF_UP) == -1)
+                                &timet, &us, _PyTime_ROUND_HALF_EVEN) == -1)
         return NULL;
 
     return datetime_from_timet_and_us(cls, f, timet, (int)us, tzinfo);