[Patch #1599256 from David Watson] check that os.fsync is available before using it

Lib/mailbox.py

@@ -1887,7 +1887,8 @@ def _create_temporary(path):
 def _sync_flush(f):
     """Ensure changes to file f are physically on disk."""
     f.flush()
-    os.fsync(f.fileno())
+    if hasattr(os, 'fsync'):
+        os.fsync(f.fileno())
 
 def _sync_close(f):
     """Close file f, ensuring all changes are physically on disk."""