Resolve "Use tox for testing multiple Python versions"

What does this MR do?

This MR changes the CI to use tox, which makes it convenient for testing dantro with multiple Python versions: Python 3.6 and 3.7 – both working 🎉

Also, testing now becomes possible via the shorter

$ tox -e py36

command, which invokes the test. If your locally available python version is 3.7, choose py37 as test environment in the command.

Can this MR be accepted?

• Adjusted the CI
• Adjusted the readme
• Pipeline passes
• Changelog entry added
• Approved by @herdeanu

Anything to double-check?

Nope.

Related issues

Closes #19 (closed)