use tox test runner

[brianray]

https://tox.readthedocs.io/en/latest/

[brianray]

@terrytangyuan good to get your feedback on tox

[brianray]

Little unclear if we need to put Python version in requirements.txt https://github.com/brianray/data-describe/blob/master/requirements.txt#L1
I suggest we also take that out.

TODO:

• create tox.ini
• add python instructions to README (if not there)
• remove explicit "python" version in requirements.txt

[terrytangyuan]

No, we should not have Python version in requirements.txt. However, we should explicitly specify the range of Python versions we support, an example here: https://github.com/tensorflow/io/blob/6505a6e6a6fd4e20bbdc09831a1489f4d26dbed2/setup.py#L137

Regarding tox, a couple of thoughts:

• Tox is definitely good for local testing.
• We are already building automated testing via GitHub Actions and can run multiple builds for different Python versions.
• If we decide to run only one build on GitHub Actions via tox for multiple Python versions, tox might be more efficient as it might be able to leverage cache across various tests in a single build. GitHub Actions have ability to store cache as well but there might still be some additional overhead to spin up multiple builds.