python3: Transition to mox3 instead of mox

In order to make the transition to mox free world,
mox needs to replaced with mox3 which is python3 compatible.

Change-Id: Id18928c7ac93b510a4867acb03896899fe1c13b0
Signed-off-by: Chuck Short <chuck.short@canonical.com>
Merge "python3: Transition to mox3 instead of mox"