Add a blurb for PEP 553.

Misc/NEWS.d/next/Core and Builtins/2017-09-05-14-19-02.bpo-31353.oGZUeJ.rst

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+PEP 553 - Add a new built-in called ``debug()`` which calls
+``sys.debughook()``.  By default this imports ``pdb`` and calls
+``pdb.set_trace()``, but users may override ``sys.debughook()`` to call
+whatever debugger they want.  The original value of the hook is saved in
+``sys.__debughook__``.