Revisit managing nox dependencies with poetry

[stpotter16]

https://cjolowicz.github.io/posts/hypermodern-python-03-linting/#managing-dependencies-in-nox-sessions-with-poetry

Poetry dumps out a constraints file that has extras configured coverage[toml] which is invalid in a constraints file.

Ways around this

• Don't use poetry dependencies in nox (not great)
• Ditch coverage all together (also not great)
• Split coverage into it's own dependency group so it can be ditched (maybe would work, but would that break tests?)
• Only specify coverage, but have install toml as extras