Keep file descriptors with Popen (for `is_windows`)

Closing file descriptors makes envdir impossible to use with chaussette,
and I cannot see why that should get done - only that it has been
changed to `not is_windows` for some reason.

Fixes #16
Merge pull request #19 from blueyed/do-not-close-fds

Keep file descriptors with Popen (for `is_windows`)