Calling `PyEval_InitThreads` has been deprecated in Python 3.7 and the
function itself will be removed in Python 3.11. The current check
guards this function behind a version check that only happens at compile
time.
This in turn leads to crashes when run on Python 3.6, as the necessary
initialization for `PyEval_ReleaseThread` did not take place.
This commit ensures the manual initialization takes place based on the
runtime version of Python and avoids loading the associated symbols
on Python 3.9 or later.
24a123119a