Stop calling PyEval_InitThreads

PyEval_InitThreads is depricated, since 3.7 it is not necessary to call
it.
Ref: https://docs.python.org/3/c-api/init.html#c.PyEval_InitThreads

Author: dkropachev

cassandra/io/libevwrapper.c

@@ -667,9 +667,11 @@ initlibevwrapper(void)
     if (PyModule_AddObject(module, "Timer", (PyObject *)&libevwrapper_TimerType) == -1)
         INITERROR;
 
+    #if PY_VERSION_HEX < 0x030700F0
     if (!PyEval_ThreadsInitialized()) {
         PyEval_InitThreads();
     }
+    #endif
 
 #if PY_MAJOR_VERSION >= 3
     return module;