news: use literal markup for undocumented unittest.util function

Sphinx cannot resolve :func: references to unittest.util since it is
not part of the public documented API. Use double backticks instead.

Author: stefanzetzsche

Misc/NEWS.d/next/Library/2026-03-05-14-13-10.gh-issue-145546.3tnlxx.rst

@@ -1,4 +1,4 @@
-Fix :func:`unittest.util.sorted_list_difference` to deduplicate remaining
+Fix ``unittest.util.sorted_list_difference()`` to deduplicate remaining
 elements when one input list is exhausted before the other. Previously,
 duplicates in the tail were included in the output despite the documented
 guarantee that "Duplicate elements in either input list are ignored."