[issue37788] fix for bpo-36402 (threading._shutdown() race condition) causes reference leak

[Roundup Robot]

Change by Roundup Robot <devnull at psf.upfronthosting.co.za>:


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pull_requests: +25992
pull_request: https://github.com/python/cpython/pull/27473

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue37788>
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