Update tox config

An updated tox config is needed to deal with accidental installs of
pre-release software we don't want. Additionally, this update should
make running tox tests quicker due to skipping the sdist.

Closes-Bug: 1233357
Change-Id: I814850d7e108a3502297bbbdda506e260d2a86cb
Add tests dir to the coverage omit list

Closes-Bug: 1233359
Change-Id: Ie337bb09f299ba1874870fa0191744bf10daa645
Merge "Update tox config"
Merge "Add tests dir to the coverage omit list"