I didn't investigate deeply (I just happened to stumble on the issue while figuring out the implementation), but it might be needed in older versions too. The loading process instantiates a `threading.RLock`, but from a source code file that doesn't import that (because it's in a file that has to bootstrap the import process). I'm not entirely sure how `threading` is supposed to get imported normally. The other complicating factor is that I was testing this at the REPL, and 3.13 has a new REPL implementation which might change how those initial libraries are brought in.