Issue #28979: Fix What's New in Python 3.6, dict

The new dict implementation is not faster, but more compact.

Patch written by Brendan Donegan.
(grafted from 181453f9a0c424212f0f6ddca2b9065c15689d7c)

Doc/whatsnew/3.6.rst

@@ -77,7 +77,7 @@ New library modules:
 CPython implementation improvements:
 
 * The :ref:`dict <typesmapping>` type has been reimplemented to use
-  a :ref:`faster, more compact representation <whatsnew36-compactdict>`
+  a :ref:`more compact representation <whatsnew36-compactdict>`
   similar to the `PyPy dict implementation`_.  This resulted in dictionaries
   using 20% to 25% less memory when compared to Python 3.5.