[3.13] GH-65961: Document the deprecation of `__package__` and `__cached__` (GH-12437...

[3.13] GH-65961: Document the deprecation of `__package__` and `__cached__` (GH-124377) (#124380)

* GH-65961: Document the deprecation of `__package__` and `__cached__` (GH-124377)

The code changes for warning related to `__package__` landed in Python 3.12. `__cached__` doesn't have any changes as it isn't used but only set by the import system.
(cherry picked from commit 67201ad53ff11576c69a9b762540b77128285f8d)

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Co-authored-by: Brett Cannon <brett@python.org>
Co-authored-by: Barry Warsaw <barry@python.org>