[issue32604] Expose the subinterpreters C-API in Python for testing use.

[STINNER Victor]

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


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pull_requests: +19393
pull_request: https://github.com/python/cpython/pull/20089

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue32604>
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