Update pyproject.toml to add flake8 and tox dependencies

[fjohnson]

The documentation lists flake8 and tox as requirements for testing (and hence contributing). We could add these as tool.poetry.dev-dependencies to the pyproject.toml file.

Are we interested in doing this? Are flake8 and tox actually going to be used? I tried flake8 which worked as there is a single test defined. Tox did not work as there is no configuration present, and this would need to be created as well.