Explicitly close stdout/stderr pipes in util.check_output
Explicitly close stdout/stderr pipes in util.check_output

Previously, these pipes were closed only when the Popen object was gc'd,
which on PyPy can happen too late so that the process runs out of pipes.

qwhelan/asv,qwhelan/asv,qwhelan/asv,pv/asv,spacetelescope/asv,spacetelescope/asv,qwhelan/asv,spacetelescope/asv,airspeed-velocity/asv,airspeed-velocity/asv,airspeed-velocity/asv,pv/asv,spacetelescope/asv,airspeed-velocity/asv,pv/asv,pv/asv