Add a NEWS entry.

Author: ericsnowcurrently

Misc/NEWS.d/next/Core and Builtins/2019-05-09-11-37-28.bpo-36854.dpBVTw.rst

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+Move GC runtime state from _PyRuntimeState to PyInterpreterState.  As part
+of this, Python threads (created by the threading module) are marked as
+"deleted" earlier in finalization.  That marker is how joining threads get
+unblocked.  It was happening in `PyThreadState_Delete()` but now it will
+happen in `PyThreadState_Clear()`.  This is necessary to ensure that the
+callback gets called *before* much interpreter/runtime finalization happens.
+(This change could impact daemon threads, but we already can't guarantee
+behavior for those once finalization starts.)