Misc/NEWS.d/next/Core and Builtins/2018-04-03-00-58-41.bpo-33205.lk2F3r.rst

Change dict growth function from ``round_up_to_power_2(used*2+hashtable_size/2)`` to
``round_up_to_power_2(used*3)``.  Previously, dict is shrinked only when ``used == 0``.
Now dict has more chance to be shrinked.