Add support to tox.

[jcfaracco]

Is it worth for this project?

https://tox.wiki/en/latest/

[Eve-ning]

Not sure about using it, unless there's a use-case that's outside the following:

• pytest handles the testing
• poetry handles the requirements.txt + setup.py
• GitHub Actions, if enabled, will automatically run the tests, unfortunately it's blocked by Implement CI/CD #21

Seems like there's much overlap in using tox?

[jcfaracco]

I don't recall immediately, but tox executes flake8, pylint and pytest.by default using a virtualenv environment, but I agree. We are still stuck with the issue above.