[ci] Adding tox

[john-bodley]

This PR adds tox a common Python testing environment. Note I also added a requirements.txt file (used in tox) which freezes all the dependencies (including the extras) which means we're no longer skipping certain tests or mocking the Tornado logic.

In future PRs I plan to:

• Replace flake8 with black, isort, and pylint.
• Add typing (with enforcement) via mypy.

to: @betodealmeida @mistercrunch

[MSadeghzadehG]

I can't see how these changes could resolve missing tabulate module problem?

[john-bodley]

@MSadeghzadehG if you look in requirements.txt (which in autogenerated from requirements.in using pip-compile) you can see a line item for the tabulate package.

[MSadeghzadehG]

@john-bodley Nice.

[john-bodley]

@mistercrunch I addressed your comment.

[SongYunSeop]

Is it still working in progress?

[john-bodley]

@SongYunSeop this PR is ready for review.

[mistercrunch]