Support for `nox` in place of `tox`

`nox` is a tool similar to `tox` that allows for running tests in multiple Python environments. See here for more info: https://nox.thea.codes/en/stable/index.html. It would be great if the templates could support `nox` as a test runner as well. I'm not sure if this is as simple as just substituting one command for another or if it would be more "interesting" than that.