It doesn't look like the fix for issue #16133 fixed this problem, or perhaps it only fixed it for asynchat but not asyncore.

I have attached a patch (against python 2, since this is where I needed it fixed).  The patch treats WSA flavors of all errno values in the same way that the non-WSA flavors were treated before.

It is the intent that no direct references to errno values persist after applying this patch.  The patch fixed my particular issue, but I certainly haven't tested every new error condition.