bpo-33205: dict: Change GROWTH_RATE to `used*3` (GH-6350)

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.

Objects/dictobject.c

@@ -396,17 +396,16 @@ dk_set_index(PyDictKeysObject *keys, Py_ssize_t i, Py_ssize_t ix)
  */
 
 /* GROWTH_RATE. Growth rate upon hitting maximum load.
- * Currently set to used*2 + capacity/2.
+ * Currently set to used*3.
  * This means that dicts double in size when growing without deletions,
  * but have more head room when the number of deletions is on a par with the
- * number of insertions.
- * Raising this to used*4 doubles memory consumption depending on the size of
- * the dictionary, but results in half the number of resizes, less effort to
- * resize.
+ * number of insertions.  See also bpo-17563 and bpo-33205.
+ *
  * GROWTH_RATE was set to used*4 up to version 3.2.
  * GROWTH_RATE was set to used*2 in version 3.3.0
+ * GROWTH_RATE was set to used*2 + capacity/2 in 3.4.0-3.6.0.
  */
-#define GROWTH_RATE(d) (((d)->ma_used*2)+((d)->ma_keys->dk_size>>1))
+#define GROWTH_RATE(d) ((d)->ma_used*3)
 
 #define ENSURE_ALLOWS_DELETIONS(d) \
     if ((d)->ma_keys->dk_lookup == lookdict_unicode_nodummy) { \