[issue35134] Add a new Include/cpython/ subdirectory for the "CPython API" with implementation details

[STINNER Victor]

Change by STINNER Victor <vstinner at python.org>:


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pull_requests: +27256
pull_request: https://github.com/python/cpython/pull/28968

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35134>
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