gh-150175: Add news entry for ThreadingMock race condition fix

Author: saisneha196

Misc/NEWS.d/next/Library/2026-05-21-fix-threadingmock-race.rst

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+Fix race condition in :class:`unittest.mock.ThreadingMock` where
+concurrent calls could lose increments to :attr:`~unittest.mock.Mock.call_count`
+and other attributes. Fixed by overriding ``_increment_mock_call`` in
+:class:`~unittest.mock.ThreadingMixin` with proper lock protection.