add deprecation changelog for py library vendoring

changelog/10396.deprecation.rst

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+pytest no longer depends on the ``py`` library.  ``pytest`` provides a vendored copy of ``py.error`` and ``py.path`` modules but will use the ``py`` library if it is installed.  If you need other ``py.*`` modules, continue to install the deprecated ``py`` library separately, otherwise it can usually be removed as a dependency.