Proposal: add tox by default

Now that `pytest-runner` was deprecated, tox have became more importantly to make sure tests happen in isolation and account for the effects of packaging. So I think it would make sense to add it by default (and have instead a `--no-tox` option).

This is not needed (specially since v4 will allow we to save this as a default configuration), but might incentivate people to run better tests...