> Thanks for the tip. I added the unit test and uploaded my final patch
> (which includes all changes).

A couple comments (note that I'm not entitled to accept or commit a patch, so feel free to ignore them if I'm just being a pain):
- "the maximum value is system-dependent (usually 5)"
I would remove this "usually 5" part: since the kernel silently adjusts the value, it shouldn't be of interest to the programmer. Furthermore, this was probably true a decade ago, but nowadays it doesn't hold anymore, Linux for example allows a much higher limit
- port = support.bind_port(srv)
Since you don't care about which port you get bound to, you could discard the port value, or even call srv.bind((HOST, 0)) directly
- I'm not sure that creating a new SocketListenBacklog0Test TestCase just for this check is necessary. Since it's so short, you could just add a new test to e.g. GeneralModuleTests

> Is it ok to remove the files I uploaded previously? 

It's probably a good idea, so there won't be any confusion with "stale" patches and test scripts.