[issue39573] Make PyObject an opaque structure in the limited C API

[Dong-hee Na]

Change by Dong-hee Na <donghee.na92 at gmail.com>:


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pull_requests: +17861
pull_request: https://github.com/python/cpython/pull/18488

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39573>
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