tox-travis: Integrate tox and Travis

tox-travis is a plugin for tox that simplifies the setup between tox and Travis.

Usage

Configure the Python versions to test with in .travis.yml, and install tox-travis with pip:

language: python
python:
  - "3.6"
  - "3.7"
install: pip install tox-travis
script: tox

tox will only run the py36 or py37 env (or envs that have a factor that matches) as appropriate for the version of Python that is being run by each Travis job.