This OSError(5) happens when we tries to read from pty after data runs out.
So simple_test_2.py fails with same error even if we don't use io module.

Modules/posixmodule.c (posix_read) simply calls read(2) once, but io module

    while avail < n:
        chunk = self.raw.read(wanted)
        if chunk in empty_values:
            nodata_val = chunk
            break
        avail += len(chunk)
        chunks.append(chunk)

chunk is shorter than wanted (data runs out), but not empty, so io
module's read tries to read again => error happens.

I said this is io module's bug, but now I'm not sure.