bpo-36710: Fix compiler warning on PyThreadState_Delete() (GH-12962)

_PyThreadState_Delete() has no return value.
diff --git a/Python/pystate.c b/Python/pystate.c
index f964f49..e9c4c7d 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -826,7 +826,7 @@
 void
 PyThreadState_Delete(PyThreadState *tstate)
 {
-    return _PyThreadState_Delete(&_PyRuntime, tstate);
+    _PyThreadState_Delete(&_PyRuntime, tstate);
 }