Add NEWS file.

Author: nascheme

Misc/NEWS.d/next/Core_and_Builtins/2025-03-14-13-08-20.gh-issue-127266._tyfBp.rst

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+In the free-threaded build, avoid data races caused by updating type slots
+or type flags after the type was initially created.  For those (typically
+rare) cases, use the stop-the-world mechanism.  Remove the use of atomics
+when reading or writing type flags.  The use of atomics is not sufficient to
+avoid races (since flags are sometimes read without a lock and without
+atomics) and are no longer required.