Add a NEWS entry.

Author: ericsnowcurrently

Misc/NEWS.d/next/Core and Builtins/2023-05-05-12-14-47.gh-issue-99113.-RAdnv.rst

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+The GIL is now (optionally) per-interpreter.  This is the fundamental change
+for PEP 684.  This is all made possible by virtue of the isolated state of
+each interpreter in the process.  The behavior of the main interpreter
+remains unchanged.  Likewise, interpreters created using
+``Py_NewInterpreter()`` are not affected.  To get an interpreter with its
+own GIL, call ``Py_NewInterpreterFromConfig()``.