The attached patch explicitly mentions that stdin/stdout/stderr are opened as binary streams when universal_newlines is False. 

I'm not convinced that adding this text will solve the confusion, subprocess has fairly complex documentation due to the rich API and it is easy to mis information when you don't read carefully. That said, the documentation for check_output (which is likely the most common way to run a command and fetch its output) is already clear about the types of the returned data, both in prose and example code.