add NEWS entry

Author: Changaco

Misc/NEWS.d/next/Library/2026-04-21-22-19-35.gh-issue-148242.unTO3m.rst

@@ -0,0 +1,4 @@
+The built-in :meth:`defaultdict.__missing__` method no longer exists. A custom
+:meth:`~object.__missing__` method should no longer insert the generated value
+into the dictionary, as this is done by the new :meth:`__getitem__` method.
+defaultdict is now safe to use with :term:`free threading`.