PROTON-595: Python Popen() failure on Windows
Reviewed at https://reviews.apache.org/r/22273/
This patch incorporates both of astitcher's review comments.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1602290 13f79535-47bb-0310-9956-ffa450edef68