Use Py_RETURN_FALSE/Py_RETURN_TRUE rather than PyBool_FromLong(0)/PyBool_FromLong(1). (#567)

diff --git a/Python/import.c b/Python/import.c
index 53c0f4d..0fc4d1d 100644
--- a/Python/import.c
+++ b/Python/import.c
@@ -240,7 +240,7 @@
 #ifdef WITH_THREAD
     return PyBool_FromLong(import_lock_thread != -1);
 #else
-    return PyBool_FromLong(0);
+    Py_RETURN_FALSE;
 #endif
 }