Add CHANGELOG

changelog/9765.bugfix.rst

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+Fixed a frustrating bug that afflicted some users with the only error being ``assert mod not in mods``. The issue was caused by the fact that ``str(Path(mod))`` and ``mod.__file__`` not necessarily produce the same string, and was being erroneously used interchangably in some places in the code.
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+This fix also broke the internal API of ``PytestPluginManager.consider_conftest`` by introducing a new parameter -- we mention this in case it is being used by external code, even if marked as *private*.