bpo-37424: Avoid a hang in subprocess.run timeout output capture (GH-14490)

bpo-37424: Avoid a hang in subprocess.run timeout output capture (GH-14490)

Fixes a possible hang when using a timeout on subprocess.run() while
capturing output. If the child process spawned its own children or otherwise
connected its stdout or stderr handles with another process, we could hang
after the timeout was reached and our child was killed when attempting to read
final output from the pipes.