gh-148395: Avoid Sphinx role nits in Tests NEWS entry

zlib._ZlibDecompressor is not a documented public class, so :class:
roles in the blurb fail Docs CI (--fail-if-new-news-nit). Use plain
double-backticks instead.

Author: Vamsi-klu

Misc/NEWS.d/next/Tests/2026-07-26-07-17-42.gh-issue-148395.rUUXvU.rst

@@ -1,5 +1,5 @@
-Add regression tests for the CVE-2026-6100 fix so a
-:class:`~bz2.BZ2Decompressor`, :class:`~lzma.LZMADecompressor`, or
-:class:`zlib._ZlibDecompressor` that raised :exc:`MemoryError` during
-:meth:`~bz2.BZ2Decompressor.decompress` cannot silently regain a dangling
-input pointer on the next call.
+Add regression tests for the CVE-2026-6100 decompressor fix so a
+``bz2.BZ2Decompressor``, ``lzma.LZMADecompressor``, or
+``zlib._ZlibDecompressor`` that raised ``MemoryError`` during
+``decompress()`` cannot silently regain a dangling input pointer on
+the next call.