Add NEWS entry for opcode tracing change. (GH-4045)

diff --git a/Misc/NEWS.d/next/Core and Builtins/2017-10-18-19-41-12.bpo-31618.liLDiS.rst b/Misc/NEWS.d/next/Core and Builtins/2017-10-18-19-41-12.bpo-31618.liLDiS.rst
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+The per-frame tracing logic added in 3.7a1 has been altered so that
+``frame->f_lineno`` is updated before either ``"line"`` or ``"opcode"``
+events are emitted. Previously, opcode events were emitted first, and
+therefore would occasionally see stale line numbers on the frame. The
+behavior of this feature has changed slightly as a result: when both
+``f_trace_lines`` and ``f_trace_opcodes`` are enabled, line events now occur
+first.