Issue 47222: subprocess.Popen() should allow capturing output and sending it to stdout and stderr

I'd like to see handlers exposes for the stdout and stderr pipe-reading threads so that I could customize what's done with the data as it's read.

I might, for instance, want to:

(1) accumulate it into a buffer;
(2) copy it onto sys.stdout or sys.stderr, respectively;
(3) send it to a logging object;

or some combination of the above, possibly all of them.