bpo-39877: Remove useless PyEval_InitThreads() calls (GH-18883)

Py_Initialize() calls PyEval_InitThreads() since Python 3.7. It's no
longer needed to call it explicitly.
diff --git a/Python/ceval.c b/Python/ceval.c
index 42f08c4..26cefea 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -345,7 +345,7 @@
     _PyRuntimeState *runtime = tstate->interp->runtime;
     struct _ceval_runtime_state *ceval = &runtime->ceval;
 
-    /* Check someone has called PyEval_InitThreads() to create the lock */
+    /* Check that _PyEval_InitThreads() was called to create the lock */
     assert(gil_created(&ceval->gil));
 
     take_gil(ceval, tstate);
@@ -541,9 +541,7 @@
 static int
 handle_signals(_PyRuntimeState *runtime)
 {
-    /* Only handle signals on main thread.  PyEval_InitThreads must
-     * have been called already.
-     */
+    /* Only handle signals on main thread */
     if (PyThread_get_thread_ident() != runtime->main_thread) {
         return 0;
     }