> Buffering is necessary for implementing the universal_newlines 

Why is that? I can see that it requires newline state tracking, and the allowance to make two read(fd, &c, 1) system calls for a single read(1) method call, in case a "\n" has to be ignored.

testproc-unbuffered.py runs to completion in 3.11 if the following statement that changes the text wrapper's chunk size is added right after creating the Popen() instance:

    if sys.version_info[0] > 2:
        process.stdout._CHUNK_SIZE = 1

The initial chunk size for a text wrapper is hard coded as 8192 bytes. For some reason the constructor has no parameter for it.