Portability issue regarding close_fds and python subprocess

This is just a heads up, that apparently specifying a value for close_fds is unsupported on Windows Python if you also redirect stdin/stdout/stderr. This is causing the test suite to fail immediately on Windows after r228130. I’m going to fix this, but I’m just posting this publicly so that people are aware of this limitation in the future. (I didn’t know about this either until this problem hit just now)