Automate Python testing across multiple Python versions. Use tox.ini to configure environments and test command.

# To run tests on all test environments:
tox

# To create a tox.ini configuration:
tox-quickstart

# To list the available environments:
tox --listenvs-all

# To run tests on a specific environment (e.g. python 3.6):
tox -e <py36>

# To force the virtual environment to be recreated:
tox --recreate -e <py27>