subprocess: close pipes/fds by using ExitStack

[giampaolo]

"In case of premature failure on X.Close() or os.close(X) the remaining pipes/fds will remain "open". Perhaps it makes sense to use contextlib.ExitStack."

This is a follow up of bpo-35537. Rationale of this change originated during code inspection in #11575 (comment).

[gpshead]

I suggest adding the following as a NEWS entry:

"An ExitStack is now used internally within subprocess.POpen to clean up pipe file handles. No behavior change in normal operation. But if closing one handle were ever to cause an exception, the others will now be closed instead of leaked."

I tried, but the blurbit web UI wouldn't let me add it to this PR.
reuse the bpo-35537 issue number, no good reason to file an issue specifically for this minor fix.