Nb-clean

This minor release migrates from Pipenv to Poetry for dependency management and packaging, and adopts Nox in CI. There are no user-facing changes.

This minor release adds a `version` subcommand that prints the version of `nb-clean`.

This minor release adds an `unconfigure-git` subcommand that removes the Git filter configuration added by `nb-clean configure-git`.

This is the initial public release of `nb-clean`.