interpreter: fix deprecation for python 3.9

ref. https://docs.python.org/3/c-api/init.html#c.PyEval_InitThreads:

> Changed in version 3.7: This function is now called by Py_Initialize(),
> so you don’t have to call it yourself anymore.

> Changed in version 3.9: The function now does nothing.

> Deprecated since version 3.9, will be removed in version 3.11.
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