Subprocess: fix stream handling

[woodruffw]

#572 provides a temporary fix by doing codepoint substitution, but #573 demonstrates that our underlying poll/buffering technique is unsound: our buffering might split a unicode sequence, resulting in invalid encodings in an otherwise valid stream.

Rather than performing the poll-buffer loop ourselves, we should probably use Popen.communicate().

[woodruffw]

Tagging @tetsuo-cpp for triage, feel free to reassign 🙂

[tetsuo-cpp]

@jleightcap This one would be also be good if you have time.