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

[tobiasgordon]

There is a potential fix in tox-travis which needs testing. We should look at trying it out.
Moving this back to the 1.4.8 to further investigate.