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.