use threading for tee, eliminate `tee: input/output error` messages
use threading for tee, eliminate `tee: input/output error` messages

chriskuehl/pip-faster,Yelp/pip-faster,Yelp/venv-update,Yelp/venv-update,Yelp/pip-faster,chriskuehl/pip-faster,Yelp/venv-update,Yelp/pip-faster