Upgrade tox dependency when tox-travis becomes compatible with tox 4.0 and later

[ni-jfitzger]

Description of issue

Due to tox-dev/tox-travis#159, we have had to cap our version of tox at 3.*.
See #1875

When tox-travis has a release available that is compatible with tox 4.0, we should update travis.yml to allow installation of tox 4.0 and later.

System report

Steps to reproduce issue