We have tox 2.9.1 (released on 2017-09-29) in CI right now and there have been 23 tox releases since then, including 1 major and 9 minor ones.
It would be nice to have a more modern one in CI.
I totally understand that upgrading tox in CI is delicate as it affects all the repositories that uses it.
An alternative approach would be to allow each project to optionally override the default tox version to require a newer one.
For example in https://gerrit.wikimedia.org/r/c/operations/software/cumin/+/508079/ I had to use a newer syntax to simply a lot the tox.ini configuration but that syntax was added in 3.0.0 and of course is failing CI.