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

[Eric Snow]

Eric Snow <ericsnowcurrently at gmail.com> added the comment:

FYI, there are a few things I need to clean up in the PR.  However, I expect that those changes will be minor relative to the the whole patch, so I wanted to get the ball rolling on a review. :)

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