Tox officially supports parallel mode as of Tox 3.7 so drop the
usage of detox. Set parallel mode to 'auto' which will run as many
threads as available CPUs.
Signed-off-by: Thanh Ha <zxiiro@gmail.com>
Change-Id: Id59ad10ed9b521a5c0ca3fef3e7e15053c2dc32c