Although many python packages use tox for testing, we shouldn't use tox in the context of Gentoo testing (and by the way, we shouldn't run coverage or flake8 either!).
This tracks packages wrongly depending on dev-python/tox for testing.
If your package depends on tox but don't ever call "tox" in the ebuild, then you can probably just remove the dependency.
If it calls tox (sometimes wrapped in a Makefile), then you should look in upstream's "tox.ini" file and run a command similar to the command described in there, and then remove dependency on tox.