Update tox.ini to stop using unverified package

As of pip 10.0, --allow-unverified is not permitted.

Use of the flag in this repo was previously used to force
installation of python-apt to accommodate certain unit tests.

The unverified package, python-apt, is no longer necessary
for test execution.

Related-Bug: #1760720

Change-Id: I4d7cb93e41256ef6b75be3a8acd83303b32ac6bd