convert to using nox for testing and github actions for CI

[jreiberkyle]

nox is basically tox with a python interface. using nox would allow us to stay in the python language while specifying our tests. nox also allows easy management of environments and supports linting and flake8 and mypy checking. Also can build docs! respx has a nice example noxfile.py

Nox and travis-ci are somehow tricky to get working together. Travis-ci is slowing down open-source builds, making the ci test loop a bit painful. nox and github actions work together nicely! So include a move to github actions for CI as a part of this work.

This solve / nullify #224