Refactor to use pyproject.toml and new build system

[bdpedigo]

This PR:

• removes requirements.txt-like files -> pyproject.toml
• removes bumpversion -> pyproject.toml (version is specified in one place only, caveclient can read it from there)
• adds optional dependency path, currently just caveclient[cv] to include a cloudvolume install

TODO:

• [ ] will need to redo github actions, but want to see how people feel about this first