[issue42990] Improve the C code for calling Python code

[Guido van Rossum]

Change by Guido van Rossum <guido at python.org>:


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nosy: +brett.cannon, petr.viktorin, rhettinger, serhiy.storchaka, vstinner, yselivanov

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue42990>
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