Update project to support Python 3.6 & 3.9 and update travis to use stages

[dgarros]

This PR update pyproject.toml and the dependencies to support Python 3.6 and python 3.9.
No changes in the code were required.

Also in the process, I did some cleanup

• Updated some files to align with the latest dev-standard
• Removed unused deps like request_mocks and ipython
• Refactor the travis file with stages, leveraging the native python version in travis.

[glennmatthews]

Looks like something isn't quite right with the pyproject.toml changes versus the poetry.lock changes:

Warning: The lock file is not up to date with the latest changes in pyproject.toml. You may be getting outdated dependencies. Run update to update them.

[glennmatthews]

One more mypy fix needed, otherwise looks good to me. Thanks!