Tox is based on the concept of environments. The goal of Tox is to prepare those environments where it will run the commands provided. Usually, those environments are meant for testing (running tests in different conditions) and the most common kind of environments are those that use different Python versions. But in theory, it is possible to create a different environment for any other purpose. For example, we frequently create an environment where project documentation is built.
To add further environments to Tox, it's sufficient to list them inside the envlist = option. To configure two environments that test our project against both Python 2.7 and Python 3.7, we can set envlist to both py37 and py27:
[tox]
setupdir = ./src
envlist = py27, py37
If we run tox again, we will see that it will now test our project on two different environments, one made for Python 2.7 and one for Python 3.7:
$ tox
GLOB sdist-make: ./src/setup.py
py27 create: ...