Add a NEWS entry.

Author: ericsnowcurrently

Misc/NEWS.d/next/Core and Builtins/2023-05-05-13-18-56.gh-issue-99113.hT1ajK.rst

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+Multi-phase init extension modules may now indicate that they support
+running in subinterpreters that have their own GIL.  This is done by using
+``Py_MOD_PER_INTERPRETER_GIL_SUPPORTED`` as the value for the
+``Py_mod_multiple_interpreters`` module def slot.  Otherwise the module, by
+default, cannot be imported in such subinterpreters.  (This does not affect
+the main interpreter or subinterpreters that do not have their own GIL.)  In
+addition to the isolation that multi-phase init already normally requires,
+support for per-interpreter GIL involves one additional constraint:
+thread-safety.  If the module has external (linked) dependencies and those
+libraries have any state that isn't thread-safe then the module must do the
+additional work to add thread-safety.  This should be an uncommon case.