Improve setup and docs

[fraser-langton]

Just adding a bit more docs on getting setup, just feedback from a first timer setting up the repo :)

I went with a basic requirements-dev.txt to hold the requirements in one spot (updated tox to reference them as well) - but maybe consider poetry?

I also tweaked the black, isort, mypy configs so they all run using <command> . (and only runs on project files) - for me I think that's kind of convention for those tools.

I also ran isort, and added to tox.ini

[seddonym]

Thanks for looking at this!

The contributing docs are indeed in need of a bit of love. Thanks for improving them.

Looks like the checks aren't all passing though.

[seddonym]

Much nicer, thanks!