> - would improve POSIX compatibility, it mimics what os.pipe()
> does on those OS

I disagree.

On Windows fds can only be inherited if you start processes using the spanwn*() family of functions.  If you start them using CreateProcess() then the underlying *handles* are inherited, but the *fds* are not.

In Python 2, os.spawn*() used spawn*(), so making os.pipe() return inheritable fds would have made some sense.  But in Python 3 os.spawn*() is implemented using subprocess/CreateProcess so fds will NOT be inherited (even if the wrapped handles are).

Note that subprocess *does* know how to redirect the standard streams to fds returned by os.pipe().

So for Python 3 I don't think there is any point in changing things.