tox-dev/tox ? reverse-engineered prompt

Build me a Python command line tool like tox that makes it easy to run tests and other tasks in isolated virtual environments.

I want to be able to define a few different environments, like different Python versions or dependency sets, then run the same test command in each one without having to set everything up by hand. It should also help check that a package installs and builds correctly in those environments, and make it simple to use in continuous integration so I can reduce all the repetitive setup in my CI jobs.

Please make it feel polished and practical, with clear commands, good defaults, and helpful output when something fails. If you need to, look up current docs online for best practices around Python packaging and virtual environments.