Revert change 0eb8c182131e:

"""Issue #23517: datetime.timedelta constructor now rounds microseconds to
nearest with ties going away from zero (ROUND_HALF_UP), as Python 2 and Python
older than 3.3, instead of rounding to nearest with ties going to nearest even
integer (ROUND_HALF_EVEN)."""

datetime.timedelta uses rounding mode ROUND_HALF_EVEN again.